xfs: refactor default quota grace period setting code

Refactor the code that sets the default quota grace period into a helper
function so that we can override the ondisk behavior later.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Amir Goldstein <amir73il@gmail.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Allison Collins <allison.henderson@oracle.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>

fs/xfs/libxfs/xfs_format.h

@@ -1209,6 +1209,11 @@ static inline void xfs_dinode_put_rdev(struct xfs_dinode *dip, xfs_dev_t rdev)
  * been reached, and therefore no expiration has been set.  Therefore, the
  * ondisk min and max defined here can be used directly to constrain the incore
  * quota expiration timestamps on a Unix system.
+ *
+ * The grace period for each quota type is stored in the root dquot (id = 0)
+ * and is applied to a non-root dquot when it exceeds the soft or hard limits.
+ * The length of quota grace periods are unsigned 32-bit quantities measured in
+ * units of seconds.  A value of zero means to use the default period.
  */
 
 /*
@@ -1223,6 +1228,14 @@ static inline void xfs_dinode_put_rdev(struct xfs_dinode *dip, xfs_dev_t rdev)
  */
 #define XFS_DQ_LEGACY_EXPIRY_MAX	((int64_t)U32_MAX)
 
+/*
+ * Default quota grace periods, ranging from zero (use the compiled defaults)
+ * to ~136 years.  These are applied to a non-root dquot that has exceeded
+ * either limit.
+ */
+#define XFS_DQ_GRACE_MIN		((int64_t)0)
+#define XFS_DQ_GRACE_MAX		((int64_t)U32_MAX)
+
 /*
  * This is the main portion of the on-disk representation of quota information
  * for a user.  We pad this with some more expansion room to construct the on