set zfs_arc_shrinker_limit to 0 by default

zfs_arc_shrinker_limit was introduced to avoid ARC collapse due to
aggressive kernel reclaim. While useful, the current default (10000) is
too prone to OOM especially when MGLRU-enabled kernels with default
min_ttl_ms are used. Even when no OOM happens, it often causes too much
swap usage.

This patch sets zfs_arc_shrinker_limit=0 to not ignore kernel reclaim
requests. ARC now plays better with both kernel shrinker and pagecache
but, should ARC collapse happen again, MGLRU behavior can be tuned or
even disabled.

Anyway, zfs should not cause OOM when ARC can be released.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Signed-off-by: Gionatan Danti <g.danti@assyoma.it>
Closes #16909

module/os/linux/zfs/arc_os.c

@@ -63,7 +63,7 @@
  * practice, the kernel's shrinker can ask us to evict up to about 4x this
  * for one allocation attempt.
  *
- * The default limit of 10,000 (in practice, 160MB per allocation attempt
+ * For example a value of 10,000 (in practice, 160MB per allocation attempt
  * with 4K pages) limits the amount of time spent attempting to reclaim ARC
  * memory to less than 100ms per allocation attempt, even with a small
  * average compressed block size of ~8KB.
@@ -71,7 +71,7 @@
  * See also the comment in arc_shrinker_count().
  * Set to 0 to disable limit.
  */
-static int zfs_arc_shrinker_limit = 10000;
+static int zfs_arc_shrinker_limit = 0;
 
 /*
  * Relative cost of ARC eviction, AKA number of seeks needed to restore evicted