ARC: Notify dbuf cache about target size reduction

ARC target size might drop significantly under memory pressure,
especially if current ARC size was much smaller than the target.
Since dbuf cache size is a fraction of the target ARC size, it
might need eviction too.  Aside of memory from the dbuf eviction
itself, it might help ARC by making more buffers evictable.

Reviewed-by: Tony Hutter <hutter2@llnl.gov>
Reviewed-by: Ameer Hamza <ahamza@ixsystems.com>
Signed-off-by:	Alexander Motin <mav@FreeBSD.org>
Sponsored by:	iXsystems, Inc.
Closes #17314
(cherry picked from commit 89a8a91582)

include/sys/dbuf.h

@@ -446,6 +446,7 @@ int dbuf_dnode_findbp(dnode_t *dn, uint64_t level, uint64_t blkid,
 
 void dbuf_init(void);
 void dbuf_fini(void);
+void dbuf_cache_reduce_target_size(void);
 
 boolean_t dbuf_is_metadata(dmu_buf_impl_t *db);