xfs: adjust the interface of xfs_qm_vop_dqalloc()

There may be not a parent inode or a name for O_TMPFILE support, but will pass
a struct xfs_mount to xfs_qm_vop_dqalloc(). So its interface need to be
adjusted in order that O_TMPFILE creation function can also use it.

Signed-off-by: Zhi Yong Wu <wuzhy@linux.vnet.ibm.com>
xfs: add xfs_create_tmpfile() for O_TMPFILE support

The function is used to create one O_TMPFILE file.
For more info, please refer to:
  http://oss.sgi.com/archives/xfs/2013-08/msg00339.html

Signed-off-by: Zhi Yong Wu <wuzhy@linux.vnet.ibm.com>
xfs: add a new method xfs_vn_tmpfile()

Add a new O_TMPFILE method to VFS inteface.
For more info, please refer to:
  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>