Fix socket poll() deadlock

In commit 7ecbf29f I added to the
poll_install() stage of poll() a check of the current state of the file -
to avoid the sleep if the file became ready before we managed to "install"
its poll request.

However, I wrongly believed it was necessary to put this check inside
the FD_LOCK together with the request installation. In fact, it doesn't
need to be in the same lock - all we need is for the check to happen
*after* the installation. The call to fo_poll() doesn't need to be in
the same FD_LOCK or even in an FD_LOCK at all.

Moreover, as it turns out, it must NOT be in an FD_LOCK() because this
results in a deadlock when polling sockets, caused by two different
code paths taking locks in opposite order:

1. Before this fix, poll() took FD_LOCK and called fo_poll() which
   called sopoll_generic() which took a SOCKBUF_LOCK

2. In the wake path, SOCKBUF_LOCK was taken, then so_wake_poll()
   is called which calls poll_wake() which takes FD_LOCK.