f2fs: make local functions static

Fixes the following sparse warnings:

fs/f2fs/segment.c:887:6: warning:
 symbol '__check_sit_bitmap' was not declared. Should it be static?
fs/f2fs/segment.c:1327:6: warning:
 symbol 'f2fs_wait_discard_bio' was not declared. Should it be static?
fs/f2fs/super.c:1661:5: warning:
 symbol 'f2fs_get_projid' was not declared. Should it be static?

Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Reviewed-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>