Convert fdLock to an RWMutex and protect the whole transaction

...against concurrent closes.

The testcase

	(set -e; while true; do truncate -s $RANDOM b; done) &
	(set -e; while true; do truncate -s $RANDOM b; done) &

uncovered lots of unnecessary RMW failures that were the result
of concurrent closes.

With this patch, the only remaining error is "Truncate on forgotten file"
that is probably caused by a problem in the go-fuse lib
( https://github.com/hanwen/go-fuse/issues/95 )

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