quota: Wire up ->quota_{enable,disable} callbacks into Q_QUOTA{ON,OFF}

Make Q_QUOTAON / Q_QUOTAOFF quotactl call ->quota_enable /
->quota_disable callback when provided. To match current behavior of
ocfs2 & ext4 we make these quotactls turn on / off quota enforcement for
appropriate quota type.

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

diff --cc fs/quota/quota.c
Simple merge

diff --cc include/linux/quotaops.h
Simple merge