MFC 338408: Don't directly dereference a user pointer in the VPD ioctl.

The PCIOCLISTVPD ioctl on /dev/pci is used to fetch a list of VPD
key-value pairs for a specific PCI function. It is used by
'pciconf -l -V'. The list is stored in a userland-supplied buffer as
an array of variable-length structures where the key and data length
are stored in a fixed-size header followed by the variable-length
value as a byte array. To facilitate walking this array in userland,
<sys/pciio.h> provides a PVE_NEXT() helper macro to return a pointer
to the next array element by reading the the length out of the current
header and using it to compute the address of the next header.

To simplify the implementation, the ioctl handler was also using
PVE_NEXT() when on the user address of the user buffer to compute the
user address of the next array element. However, the PVE_NEXT() macro
when used with a user address was reading the value's length by
indirecting the user pointer. The value was ready after the current
record had been copied out to the user buffer, so it appeared to work
on architectures where user addresses are directly dereferencable from
the kernel (all but powerpc and i386 after the 4:4 split). The recent
enablement of SMAP on amd64 caught this violation however. To fix,
add a variant of PVE_NEXT() for use in the ioctl handler that takes an
explicit value length.

MFC 303881: Reliably return PCI_GETCONF_LAST_DEVICE from PCIOCGETCONF.

Previously the loop in PCIIOCGETCONF would terminate as soon as it
found enough matches. Now it will continue iterating through the
PCI device list and only terminate if it finds another matching device
for which it has no room to store a conf structure. This means that
PCI_GETCONF_LAST_DEVICE is reliably returned when the number of
matching devices is equal to the number of slots in the matches
buffer. For example, if a program requests the conf structure for a
single PCI function with a specified domain/bus/slot/function it will
now get PCI_GETCONF_LAST_DEVICE instead of PCI_GETCONF_MORE_DEVS.

While here, simplify the loop conditional a bit more by explicitly
breaking out of the loop if copyout() fails and removing a redundant
i < pci_numdevs check.

Sponsored by: Chelsio Communications

MFC 295813,295816: Remove redundant check for "(dinfo != NULL)".

MFC 260926:
Add support for displaying VPD for PCI devices via pciconf.
- Store the length of each read-only VPD value since not all values are
guaranteed to be ASCII values (though most are).
- Add a new pciio ioctl to fetch VPD for a single PCI device. The values
are returned as a list of variable length records, one for the device
name and each keyword.
- Add a new -V flag to pciconf's list mode which displays VPD data for
each device.

MFC 260926:
Add support for displaying VPD for PCI devices via pciconf.
- Store the length of each read-only VPD value since not all values are
guaranteed to be ASCII values (though most are).
- Add a new pciio ioctl to fetch VPD for a single PCI device. The values
are returned as a list of variable length records, one for the device
name and each keyword.
- Add a new -V flag to pciconf's list mode which displays VPD data for
each device.