gfs2: Another gfs2_walk_metadata fix

[ Upstream commit 566a2ab3c9005f62e784bd39022d58d34ef4365c ]

Make sure we don't walk past the end of the metadata in gfs2_walk_metadata: the
inode holds fewer pointers than indirect blocks.

Slightly clean up gfs2_iomap_get.

Fixes: a27a0c9b6a20 ("gfs2: gfs2_walk_metadata fix")
Cc: stable@vger.kernel.org # v5.3+
Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>
Signed-off-by: Bob Peterson <rpeterso@redhat.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>