geli(8): Do not report error on resize to the same size.

Just validate the old metadata and exit. Originally the check was
added to not thash the only copy of metadata, but we can achieve the
same just by skipping the writing/trashing. The metadata validation
should protect user from wrongly specifying new size instead of old.

MFC after: 1 month
Sponsored by: iXsystems, Inc.

(cherry picked from commit c7cf100aafb4cb881e05a5153de152907f6c07f3)