Add ZAP shrinking support

This allows ZAPs to shrink. When there are two empty sibling leafs,
one of them is collapsed and its storage space is reused.
This improved performance on directories that at one time contained
a large number of files, but many or all of those files have since
been deleted.

This also applies to all other types of ZAPs as well.

Sponsored-by: iXsystems, Inc.
Sponsored-by: Klara, Inc.
Signed-off-by: Alexander Stetsenko <[email protected]>

man/man4/zfs.4

@@ -16,7 +16,7 @@
 .\" own identifying information:
 .\" Portions Copyright [yyyy] [name of copyright owner]
 .\"
-.Dd January 9, 2024
+.Dd February 14, 2024
 .Dt ZFS 4
 .Os
 .
@@ -564,9 +564,8 @@ However, this is limited by
 Maximum micro ZAP size.
 A micro ZAP is upgraded to a fat ZAP, once it grows beyond the specified size.
 .
-.It Sy zfetch_hole_shift Ns = Ns Sy 2 Pq uint
-Log2 fraction of holes in speculative prefetch stream allowed for it to
-proceed.
+.It Sy zap_shrink_enabled Ns = Ns Sy 1 Ns | Ns 0 Pq int
+If set, adjacent empty ZAP blocks will be collapsed, reducing disk space.
 .
 .It Sy zfetch_min_distance Ns = Ns Sy 4194304 Ns B Po 4 MiB Pc Pq uint
 Min bytes to prefetch per stream.