core: Remove taking of events lock inside usbi_fd_notification()

core: Remove taking of events lock inside usbi_fd_notification()

It is unnecessary to take the events lock when a thread needs to
interrupt an event handler to receive a change list of poll fds.
It is sufficient to simply write to the control pipe and let the
event handler be notified of this event when it polls.

Taking the events lock inside this function leads to opportunity
for deadlock in certain situations, such as a client program
running on an OS that uses fd notification on each individual
transfer. If the client program were to protect a list of transfers
with a single lock, it could deadlock if that lock were taken in
two separate threads, one which is attempting to submit a new
transfer and one which is executing inside the transfer completion
callback.

Thanks to Dmitry Fleytman and Pavel Gurvich for reporting this.

Signed-off-by: Chris Dickens <christopher.a.dickens@gmail.com>