sys: Remove $FreeBSD$: one-line .c pattern

Remove /^[\s*]*__FBSDID\("\$FreeBSD\$"\);?\s*\n/

linuxkpi: Accept NULL as a value in `linux_xarray`

Linux' XArray allows to store a NULL pointer as a value. `xa_load()`
would return NULL for both an unused index and an index set to NULL. But
it impacts `xa_alloc()` which needs to find the next available index.

However, our implementation relies on a radix tree (see `linux_radix.c`)
which does not accept NULL pointers as values. I'm not sure if this is a
limitation or a feature, so to work around this, a NULL value is
replaced by `NULL_VALUE`, an unlikely address, when we pass it to
linux_radix.

Reviewed by: emaste, manu
Approved by: emaste, manu
Differential Revision: https://reviews.freebsd.org/D38543

linuxkpi xarray: Correct expression in assertion.

Reported by: GCC -Wparantheses
Reviewed by: wulf, hselasky
Differential Revision: https://reviews.freebsd.org/D34197

LinuxKPI: Add support for XA_FLAGS_ALLOC1 xarray flag

XA_FLAGS_ALLOC1 causes allocation of xarray entries starting at 1

Required by drm-kmod 5.7

MFC after: 1 week
Reviewed by: hselasky, manu
Differential Revision: https://reviews.freebsd.org/D33293

Implement extensible arrays API using the existing radix tree implementation
in the LinuxKPI.

Differential Revision: https://reviews.freebsd.org/D25101
Reviewed by: kib @
MFC after: 1 week
Sponsored by: Mellanox Technologies