The open path for ptmx slaves is via the ptmx device. Opening them any
other way is not allowed. Vegard Nossum found that previously this was not
the case and mknod foo c 128 42; cat foo would produce nasty diagnostics
Signed-off-by: NAlan Cox <alan@redhat.com>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>