sound: rawmidi: fix checking of O_APPEND when opening MIDI device

sound: rawmidi: fix checking of O_APPEND when opening MIDI device

commit 16fb109644b5644e42ececeff644514de6f4bd03 upstream.

Commit 9a1b64caac82aa02cb74587ffc798e6f42c6170a in 2.6.30 dropped the
check that a substream must already have been opened with O_APPEND to be
able to open it a second time.

This would make it possible for a substream to be switched to append
mode, which would mean that non-atomic writes would fail unexpectedly.

Signed-off-by: Clemens Ladisch <clemens@ladisch.de>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>