sbitmap: fix all kernel-doc warnings

sbitmap: fix all kernel-doc warnings

Modify kernel-doc comments in sbitmap.h to prevent warnings:

Warning: include/linux/sbitmap.h:84 struct member 'alloc_hint' not
 described in 'sbitmap'
Warning: include/linux/sbitmap.h:151 struct member 'ws_active' not
 described in 'sbitmap_queue'
Warning: include/linux/sbitmap.h:552 No description found for
 return value of 'sbq_wait_ptr'

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