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

Orabug: 22623817

commit 4dcb8b57df3593dcb20481d9d6cf79d1dc1534be upstream.

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>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
(cherry picked from commit 8104ee7b8f8bcfad0114cdad62dd3b4aea5fcf74)
Signed-off-by: Dan Duval <dan.duval@oracle.com>