Protecting the handlers' vector is not enough. Managing the interrupts
involves other tasks that must be serialized. Mainly to handle the first
request and the last free of a VIC handler.

The VIC internal locking (vec_lock) is not useful anymore, but I do not
remove it because theoretically the VIC should be an independent bunch of
code.
Signed-off-by: Federico Vaga <federico.vaga@cern.ch>