Several improvements to ARC shrinking (#16197)

- When receiving memory pressure signal from OS be more strict
trying to free some memory.  Otherwise kernel may come again and
request much more.  Return as result how much arc_c was actually
reduced due to this request, that may be less than requested.
 - On Linux when receiving direct reclaim from some file system
(that may be ZFS) instead of ignoring request completely, just
shrink the ARC, but do not wait for eviction.  Waiting there may
cause deadlock.  Ignoring it as before may put extra pressure on
other caches and/or swap, and cause OOM if nothing help.  While
not waiting may result in more ARC evicted later, and may be too
late if OOM killer activate right now, but I hope it to be better
than doing nothing at all.
 - On Linux set arc_no_grow before waiting for reclaim, not after,
or it may grow back while we are waiting.
 - On Linux add new parameter zfs_arc_shrinker_seeks to balance
ARC eviction cost, relative to page cache and other subsystems.
 - Slightly update Linux arc_set_sys_free() math for new kernels.

Signed-off-by:	Alexander Motin <mav@FreeBSD.org>
Sponsored by:	iXsystems, Inc.
Reviewed-by: Rob Norris <rob.norris@klarasystems.com>
Reviewed-by: Tony Hutter <hutter2@llnl.gov>

man/man4/zfs.4

@@ -831,6 +831,13 @@ even with a small average compressed block size of ~8 KiB.
 The parameter can be set to 0 (zero) to disable the limit,
 and only applies on Linux.
 .
+.It Sy zfs_arc_shrinker_seeks Ns = Ns Sy 2 Pq int
+Relative cost of ARC eviction on Linux, AKA number of seeks needed to
+restore evicted page.
+Bigger values make ARC more precious and evictions smaller, comparing to
+other kernel subsystems.
+Value of 4 means parity with page cache.
+.
 .It Sy zfs_arc_sys_free Ns = Ns Sy 0 Ns B Pq u64
 The target number of bytes the ARC should leave as free memory on the system.
 If zero, equivalent to the bigger of