bgpd: fix race condition causing occasional assert

bgpd: fix race condition causing occasional assert

If a BGP message header fails validation we send a BGP NOTIFICATION from
the I/O thread. At this time we clear the output buffer, push a
NOTIFICATION and then call the manual write function for errors. But in
between the push and the write the main thread could have pushed some
other message. Thus we need to hold the lock for the duration of the
function. TOCTTOU.

Signed-off-by: Quentin Young <qlyoung@cumulusnetworks.com>