Simplify devpts_get_tty()

As pointed out by H. Peter Anvin, since the inode for the pty is known,
we don't need to look it up.

Signed-off-by: Sukadev Bhattiprolu <sukadev@us.ibm.com>
Signed-off-by: Alan Cox <alan@redhat.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>