fs: fix new kernel-doc warnings in fs/bio.c

Fix new kernel-doc warnings in fs/bio.c:

Warning(fs/bio.c:316): No description found for parameter 'bio'
Warning(fs/bio.c:316): No description found for parameter 'parent'

Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Jens Axboe <axboe@fb.com>