Linux: Report reclaimable memory to kernel as such (#16385)

Linux provides SLAB_RECLAIM_ACCOUNT and __GFP_RECLAIMABLE flags to
mark memory allocations that can be freed via shinker calls.  It
should allow kernel to tune and group such allocations for lower
memory fragmentation and better reclamation under pressure.

This patch marks as reclaimable most of ARC memory, directly
evictable via ZFS shrinker, plus also dnode/znode/sa memory,
indirectly evictable via kernel's superblock shrinker.

Signed-off-by:	Alexander Motin <mav@FreeBSD.org>
Sponsored by:	iXsystems, Inc.
Reviewed-by: Tony Hutter <hutter2@llnl.gov>
Reviewed-by: Allan Jude <allan@klarasystems.com>

include/os/freebsd/spl/sys/kmem.h

@@ -49,6 +49,7 @@ MALLOC_DECLARE(M_SOLARIS);
 #define	KM_NOSLEEP		M_NOWAIT
 #define	KM_NORMALPRI		0
 #define	KMC_NODEBUG		UMA_ZONE_NODUMP
+#define	KMC_RECLAIMABLE		0x0
 
 typedef struct vmem vmem_t;