dm btree: fix leak of bufio-backed block in btree_split_beneath error path

dm btree: fix leak of bufio-backed block in btree_split_beneath error path

[ Upstream commit 4dcb8b57df3593dcb20481d9d6cf79d1dc1534be ]

btree_split_beneath()'s error path had an outstanding FIXME that speaks
directly to the potential for _not_ cleaning up a previously allocated
bufio-backed block.

Fix this by releasing the previously allocated bufio block using
unlock_block().

Reported-by: Mikulas Patocka <mpatocka@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>
Acked-by: Joe Thornber <thornber@redhat.com>
Cc: stable@vger.kernel.org
Signed-off-by: Sasha Levin <sasha.levin@oracle.com>