lib/sbitmap: make sbitmap_get_shallow() internal

Because it's only used in sbitmap.c

Signed-off-by: Yu Kuai <yukuai3@huawei.com>
Reviewed-by: Damien Le Moal <dlemoal@kernel.org>
Reviewed-by: Jan Kara <jack@suse.cz>
Reviewed-by: Bart Van Assche <bvanassche@acm.org>
Link: https://lore.kernel.org/r/20250807032413.1469456-3-yukuai1@huaweicloud.com


Signed-off-by: Jens Axboe <axboe@kernel.dk>