powerpc: Support execute-only on all powerpc

Introduce PAGE_EXECONLY_X macro which provides exec-only rights.
The _X may be seen as redundant with the EXECONLY but it helps
keep consistency, all macros having the EXEC right have _X.

And put it next to PAGE_NONE as PAGE_EXECONLY_X is
somehow PAGE_NONE + EXEC just like all other SOMETHING_X are
just SOMETHING + EXEC.

On book3s/64 PAGE_EXECONLY becomes PAGE_READONLY_X.

On book3s/64, as PAGE_EXECONLY is only valid for Radix add
VM_READ flag in vm_get_page_prot() for non-Radix.

And update access_error() so that a non exec fault on a VM_EXEC only
mapping is always invalid, even when the underlying layer don't
always generate a fault for that.

For 8xx, set PAGE_EXECONLY_X as _PAGE_NA | _PAGE_EXEC.
For others, only set it as just _PAGE_EXEC

With that change, 8xx, e500 and 44x fully honor execute-only
protection.

On 40x that is a partial implementation of execute-only. The
implementation won't be complete because once a TLB has been loaded
via the Instruction TLB miss handler, it will be possible to read
the page. But at least it can't be read unless it is executed first.

On 603 MMU, TLB missed are handled by SW and there are separate
DTLB and ITLB. Execute-only is therefore now supported by not loading
DTLB when read access is not permitted.

On hash (604) MMU it is more tricky because hash table is common to
load/store and execute. Nevertheless it is still possible to check
whether _PAGE_READ is set before loading hash table for a load/store
access. At least it can't be read unless it is executed first.

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/4283ea9cbef9ff2fbee468904800e1962bc8fc18.1695659959.git.christophe.leroy@csgroup.eu

powerpc: Refactor permission masks used for __P/__S table and kernel memory flags

Prepare a common version of the permission masks that will be based
on _PAGE_NA, _PAGE_RO, _PAGE_ROX, _PAGE_RW, _PAGE_RWX that will be
defined in platform specific headers in later patches.

Put them in a new header pgtable-masks.h which will be included by
platforms.

And prepare a common version of flags used for mapping kernel memory
that will be based on _PAGE_RO, _PAGE_ROX, _PAGE_RW, _PAGE_RWX that
will be defined in platform specific headers.

Put them in unless _PAGE_KERNEL_RO is already defined so that platform
specific definitions can be dismantled one by one.

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/d31b59efcdb38e675479563307af891aeadf6ec9.1695659959.git.christophe.leroy@csgroup.eu