Mark functions as static in bio-integrity.c because it is not used
outside this file.

This eliminates the following warnings in bio-integrity.c:
fs/bio-integrity.c:224:5: warning: no previous prototype for ‘bio_integrity_tag’ [-Wmissing-prototypes]
Signed-off-by: Rashika Kheria <rashika.kheria@gmail.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>
Signed-off-by: Jens Axboe <axboe@fb.com>