Simplify devpts_pty_new()

devpts_pty_new() is called when setting up a new pty and would not
will not have an existing dentry or inode for the pty. So don't bother
looking for an existing dentry - just create a new one.

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>