Tune ARC memory pressure parameters (#13951)

- Setting zfs_arc_shrinker_limit=0 should make ARC more obedient to
all kernel shrinker KPI requests, that should help to avoid OOMs.

 - zfs_arc_pc_percent=300 should prevent file-backed part of page
cache from getting bigger than 1/3 of ARC, that should help with ARC
shrinking to its minimum by heavy mmap() I/O.  It is not perfect
from perspective of adapting to different workloads, but the best
we can do for now.

(cherry picked from commit 29746fa)

Co-authored-by: Alexander Motin <[email protected]>

src/freenas/usr/lib/modprobe.d/truenas.conf

@@ -1,3 +1,3 @@
 options spl spl_panic_halt=1
-options zfs zfs_default_ibs=15
+options zfs zfs_default_ibs=15 zfs_arc_shrinker_limit=0 zfs_arc_pc_percent=300
 blacklist irdma