quota: clean up quota active checks

The various quota operations check for any quota beeing active on
a superblock, and the inode not having the noquota flag.

Merge these two checks into a dquot_active check and move that
into dquot.c as that's the only place where it's needed.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jan Kara <jack@suse.cz>