xfs: Define a new function xfs_is_quota_inode()

In preparation for combined pquota/gquota support, define
a new function to check if the given inode is a quota inode.

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

diff --git a/fs/xfs/xfs_itable.c b/fs/xfs/xfs_itable.c
index 06d004d..bc92c53 100644
--- a/fs/xfs/xfs_itable.c
+++ b/fs/xfs/xfs_itable.c
@@ -43,7 +43,7 @@
 {
 	return (ino == mp->m_sb.sb_rbmino || ino == mp->m_sb.sb_rsumino ||
 		(xfs_sb_version_hasquota(&mp->m_sb) &&
-		 (ino == mp->m_sb.sb_uquotino || ino == mp->m_sb.sb_gquotino)));
+		 xfs_is_quota_inode(&mp->m_sb, ino)));
 }
 
 /*