dm-verity: correctly handle dm_bufio_client_create() failure

dm-verity: correctly handle dm_bufio_client_create() failure

If either of the calls to dm_bufio_client_create() in verity_fec_ctr()
fails, then dm_bufio_client_destroy() is later called with an ERR_PTR()
argument.  That causes a crash.  Fix this.

Fixes: a739ff3f543a ("dm verity: add support for forward error correction")
Cc: stable@vger.kernel.org
Reviewed-by: Sami Tolvanen <samitolvanen@google.com>
Signed-off-by: Eric Biggers <ebiggers@kernel.org>
Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>