block: don't select PERCPU_RWSEM

The block device doesn't use percpu rw-semaphore anymore, so don't select
it for compilation.

Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Cc: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>