drbd: Do not redefine __must_hold()

Since __must_hold() is defined in <linux/compiler_types.h>, do not
redefine it in DRBD. Compile-tested only.

Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Bart Van Assche <bart.vanassche@wdc.com>
Cc: Philipp Reisner <philipp.reisner@linbit.com>
Cc: Lars Ellenberg <lars.ellenberg@linbit.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>