Make it static, even.  And remove a stale extern of (long-gone)
nfs_xdev_mount_common() from internal.h, while we are at it.
Reviewed-by: David Howells <dhowells@redhat.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>