IAX2: fix race condition with nativebridge transfers.

1). When touching the bridgecallno, we need to lock it.

2). stop_stuff() which calls iax2_destroy_helper()
    Assumes the lock on the pvt is already held, when iax2_destroy_helper() is called.
    Thus we need to lock the bridgecallno pvt before we call stop_stuff(iaxs[fr->callno]->bridgecallno);

3).   When evaluating the state of 'callno->transferring' of the current leg,
    we can't change it to READY unless the bridgecallno is locked.
      Why, if we are interrupted by the other call leg before 'transferring = TRANSFER_RELEASED',
    the interrupt will find that it is READY and that the bridgecallno is also READY so Releases the legs.

(closes issue ASTERISK-21409)

Reported by: alecdavis
Tested by: alecdavis
alecdavis (license 585)

Review https://reviewboard.asterisk.org/r/2594/
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Merged revisions 391062 from http://svn.asterisk.org/svn/asterisk/branches/1.8
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Merged revisions 391063 from http://svn.asterisk.org/svn/asterisk/branches/11


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@391064 65c4cc65-6c06-0410-ace0-fbb531ad65f3