xfs: add O_TMPFILE support

Add two functions xfs_create_tmpfile() and xfs_vn_tmpfile()
to support O_TMPFILE file creation.

In contrast to xfs_create(), xfs_create_tmpfile() has a different
log reservation to the regular file creation because there is no
directory modification, and doesn't check if an entry can be added
to the directory, but the reservation quotas is required appropriately,
and finally its inode is added to the unlinked list.

xfs_vn_tmpfile() add one O_TMPFILE method to VFS interface and directly
invoke xfs_create_tmpfile().

Signed-off-by: Zhi Yong Wu <wuzhy@linux.vnet.ibm.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Ben Myers <bpm@sgi.com>