xfs: Start using pquotaino from the superblock.

Start using pquotino and define a macro to check if the
superblock has pquotino.

Keep backward compatibilty by alowing mount of older superblock
with no separate pquota 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>