Split out initializing the nfs4_callback structure from using it.  For
the NULL callback this gets rid of tons of pointless re-initializations.

Note that I don't quite understand what protects us from running multiple
NULL callbacks at the same time, but at least this chance doesn't make
it worse..
Signed-off-by: NChristoph Hellwig <hch@lst.de>
Reviewed-by: NJeff Layton <jlayton@primarydata.com>
Signed-off-by: NJ. Bruce Fields <bfields@redhat.com>