Fast Dedup: ZAP Shrinking

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.
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Signed-off-by: Alexander Stetsenko <alex.stetsenko@klarasystems.com>
Closes #15888