Define a new function xfs_inode_dquot()

Define a new function xfs_inode_dquot() that takes a inode pointer
and a disk quota type and returns the quota pointer for the specified
quota type.

This simplifies the xfs_qm_dqget() error path significantly.

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