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>

fs/nfsd/trace.h

@@ -408,7 +408,6 @@ TRACE_EVENT(nfsd_dirent,
 		__entry->ino = ino;
 		__entry->len = namlen;
 		memcpy(__get_str(name), name, namlen);
-		__assign_str(name, name);
 	),
 	TP_printk("fh_hash=0x%08x ino=%llu name=%.*s",
 		__entry->fh_hash, __entry->ino,