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 --git a/include/linux/quotaops.h b/include/linux/quotaops.h
index 29e3455..ff0b665 100644
--- a/include/linux/quotaops.h
+++ b/include/linux/quotaops.h
@@ -386,4 +386,6 @@
 	__dquot_free_space(inode, nr << inode->i_blkbits, DQUOT_SPACE_RESERVE);
 }
 
+unsigned int qtype_enforce_flag(int type);
+
 #endif /* _LINUX_QUOTAOPS_ */