nfsd: Return the correct number of bytes written to the file

We must allow for the fact that iov_iter_write() could have returned
a short write (e.g. if there was an ENOSPC issue).

Fixes: d890be15

 "nfsd: Add I/O trace points in the NFSv4 write path"
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>