Define a new function xfs_this_quota_on()

Create a new function xfs_this_quota_on() that takes a xfs_mount
data structure and a disk quota type and returns true if the specified
type of quota is ON in the xfs_mount data structure.

Signed-off-by: Chandra Seetharaman <sekharan@us.ibm.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Ben Myers <bpm@sgi.com>