fs/bio.c: remove nr_segs (unused function parameter)

nr_segs is no longer used in bio_alloc_map_data since c8db4448
("block: Don't save/copy bvec array anymore")

Signed-off-by: Fabian Frederick <fabf@skynet.be>
Cc: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Jens Axboe <axboe@fb.com>