libnfs: Don't perform operations on nfsfh after user callback

Since the user callback may perform operations on the nfsfh (e.g. it
might close it), all updates should be done before the user callback is
called.

Signed-off-by: Ross Lagerwall <rosslagerwall@gmail.com>
Merge pull request #97 from rosslagerwall/read-close-crash

libnfs: Don't perform operations on nfsfh after user callback