chan_dahdi.c: Lock private struct for ast_write().

chan_dahdi.c: Lock private struct for ast_write().

There is a window of opportunity for DTMF to not go out if an audio frame
is in the process of being written to DAHDI while another thread starts
sending DTMF.  The thread sending the audio frame could be past the
currently dialing check before being preempted by another thread starting
a DTMF generation request.  When the thread sending the audio frame
resumes it will then cause DAHDI to stop the DTMF tone generation.  The
result is no DTMF goes out.

* Made dahdi_write() lock the private struct before writing to the DAHDI
file descriptor.

ASTERISK-25315
Reported by John Hardin

Change-Id: Ib4e0264cf63305ed5da701188447668e72ec9abb