LinuxKPI: Chase for new "name" argument of register_shrinker()

Linux uses it only if SHRINKER_DEBUG config option is enabled. Ignore it.

Sponsored by: Serenity Cyber Security, LLC
Reviewers: manu, bz
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D42809

sys: Remove $FreeBSD$: two-line .h pattern

Remove /^\s*\*\n \*\s+\$FreeBSD\$$\n/

linuxkpi: Add `synchronize_shrinkers()`

It takes the lock and release it immediately to make sure no shrinkers
are running in parallel.

Reviewed by: manu
Approved by: manu
Differential Revision: https://reviews.freebsd.org/D38565

linuxkpi: Add `gfp_mask` field to `struct shrink_control`

bz@ asked if the KBI breakage is a concern here. My answer was that this
is the first time in the DRM drivers in Linux 5.13 (the version I'm
working on) that this structure is initialized (as a variable local to
the function in this case), so it shouldn't be a problem for the DRM
drivers.

However, I can't speak for other drivers maintained outside of the src
tree.

Reviewed by: emaste, manu
Approved by: emaste, manu
Differential Revision: https://reviews.freebsd.org/D37913

LinuxKPI: Constantly use _LINUXKPI_ prefix in include guards

MFC after: 1 week
Reviewed by: bz, emaste, hselasky, manu
Differential Revision: https://reviews.freebsd.org/D33562

linuxkpi: Add shrinker support

A driver can register a shrinker that will be called when the kernel
wants to free some memory.
Add support for that in linuxkpi and call the registered shrinkers
when the lowmem event is triggered.

Reviewed by: bz
Differential Revision: https://reviews.freebsd.org/D27728