xfs: factor prid related codes into xfs_get_initial_prid()

It will be reused by the O_TMPFILE creation function.

Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Zhi Yong Wu <wuzhy@linux.vnet.ibm.com>
xfs: add O_TMPFILE support

Add xfs_create_tmpfile() to create one O_TMPFILE file, and a new method
xfs_vn_tmpfile() to add a new O_TMPFILE method to VFS inteface.

For more info, please refer to:
  http://oss.sgi.com/archives/xfs/2013-08/msg00339.html
  http://oss.sgi.com/archives/xfs/2013-08/msg00336.html

Signed-off-by: Zhi Yong Wu <wuzhy@linux.vnet.ibm.com>
xfs: allow linkat() on O_TMPFILE files

Enable O_TMPFILE support in linkat().
For more info, please refer to:
  http://oss.sgi.com/archives/xfs/2013-08/msg00341.html

Signed-off-by: Zhi Yong Wu <wuzhy@linux.vnet.ibm.com>