lchan_fsm: add in_release_handler flag

If a release event is being handled, ignore other ricocheting release events
until that release handling has concluded.

For example, if an lchan is regularly released, it signals the lchan RTP FSM to
release, which then calls back to say "RTP is released" on termination -- this
should not trigger other state changes than the initial release intends.

Change-Id: Iec41e006b6ab9d0f618d36925341f9536353e5d8
2 files changed