md: bcache: io.c: fix a potential NULL pointer dereference

commit 5c694129c8db6d89c9be109049a16510b2f70f6d upstream.

bio_alloc_bioset returns NULL on failure. This fix adds a missing check
for potential NULL pointer dereferencing.

Signed-off-by: Kumar Amit Mehta <gmate.amit@gmail.com>
Signed-off-by: Kent Overstreet <koverstreet@google.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>