devpts: Simplify devpts_mount by using mount_nodev

Now that all of the work of setting up a superblock has been moved to
devpts_fill_super simplify devpts_mount by calling mount_nodev instead
of rolling mount_nodev by hand.

Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>