NFSD: Prevent a possible oops in the nfs_dirent() tracepoint

The double copy of the string is a mistake, plus __assign_str()
uses strlen(), which is wrong to do on a string that isn't
guaranteed to be NUL-terminated.

Fixes: 6019ce07 ("NFSD: Add a tracepoint to record directory entry encoding")
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>