brd: remove unused brd_mutex

Remove unused mutex brd_mutex. It is unused since the commit ff26956875c2
("brd: remove support for BLKFLSBUF").

Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
1 file changed