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 --cc fs/xfs/xfs_itable.c
Simple merge

diff --cc fs/xfs/xfs_qm.c
Simple merge

diff --cc fs/xfs/xfs_sb.h
Simple merge

diff --cc fs/xfs/xfs_trans_dquot.c
Simple merge