quota: Sanity-check quota file headers on load

Perform basic sanity checks of quota headers to avoid kernel crashes on
corrupted quota files.

CC: stable@vger.kernel.org
Reported-by:  <syzbot+f816042a7ae2225f25ba@syzkaller.appspotmail.com>
Reviewed-by: Andreas Dilger <adilger@dilger.ca>
Signed-off-by: Jan Kara <jack@suse.cz>