9pfs: don't use AT_EMPTY_PATH in local_set_cred_passthrough()

The name argument can never be an empty string, and dirfd always point to
the containing directory of the file name. AT_EMPTY_PATH is hence useless
here. Also it breaks build with glibc version 2.13 and older.

It is actually an oversight of a previous tentative patch to implement this
function. We can safely drop it.
Reported-by: NMark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Signed-off-by: NGreg Kurz <groug@kaod.org>
Tested-by: NMark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Reviewed-by: NEric Blake <eblake@redhat.com>

hw/9pfs/9p-local.c

@@ -349,7 +349,7 @@ static int local_set_cred_passthrough(FsContext *fs_ctx, int dirfd,
                                       const char *name, FsCred *credp)
 {
     if (fchownat(dirfd, name, credp->fc_uid, credp->fc_gid,
-                 AT_SYMLINK_NOFOLLOW | AT_EMPTY_PATH) < 0) {
+                 AT_SYMLINK_NOFOLLOW) < 0) {
         /*
          * If we fail to change ownership and if we are
          * using security model none. Ignore the error