Hopefully last part of the fix for bug 6651 - smbd SIGSEGV when breaking oplocks.

This one is subtle. There is a race condition where a signal can be
queued for oplock break, and then the file can be closed by the client
before the signal can be processed. Currently if this occurs we panic
(we can't match an incoming signal fd with a fsp pointer). Simply log
the error (at debug level 10 right now, might be too much) and then
return without processing the break request. It looks like there is
another race condition with this fix, but here's why it won't happen.
If the signal was pending (caused by a kernel oplock break from a
local file open), and the client closed the file and then re-opened
another file which happened to use the same file descriptor as the
file just closed, then theoretically the oplock break requests could
be processed on the wrong fd. Here's why this should be very rare..
Processing a pending signal always take precedence over an incoming
network request, so as long as the client close request is non-chained
then the break signal should always be harmlessly processed *before*
the open can be called. If the open is chained onto the close, and
the fd on the new open is the same as the old closed fd, then it's
possible this race will occur. However, all that will happen is that
we'll lose the oplock on this file. A shame, but not a fatal event.
Jeremy.

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