tty: hold lock across tty buffer finding and buffer filling

tty_buffer_request_room is well protected, but while after it returns,
 it releases the port->lock. tty->buf.tail might be modified
by either irq handler or other threads. The patch adds more protection
by holding the lock across tty buffer finding and buffer filling.

Signed-off-by: Alek Du <alek.du@intel.com>
Signed-off-by: Xiaobing Tu <xiaobing.tu@intel.com>
Cc: Jiri Slaby <jslaby@suse.cz>
Cc: Alan Cox <alan@lxorguk.ukuu.org.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
CRs-Fixed: 387308
Signed-off-by: Vamsi Krishna <vskrishn@codeaurora.org>
(cherry picked from commit 09236b0fbea5268bce3568d974654f8d9ddb83ec)

Change-Id: If383e12d6e65cfde1e2df2e65df8c3a4c30385ed
Signed-off-by: Sridhar Gujje <sgujje@codeaurora.org>
fix hotplug