f2fs: add f2fs_bug_on() in f2fs_quota_read()

mapping_read_folio_gfp() will return a folio, it should always be
uptodate, let's check folio uptodate status to detect any potenial
bug.

Signed-off-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>