ovl: fix missing override creds in link of a metacopy upper

Theodore Ts'o reported a v4.19 regression with docker-dropbox:
https://marc.info/?l=linux-fsdevel&m=154070089431116&w=2



"I was rebuilding my dropbox Docker container, and it failed in 4.19
 with the following error:
 ...
 dpkg: error: error creating new backup file \
              '/var/lib/dpkg/status-old': Invalid cross-device link"

The problem did not reproduce with metacopy feature disabled.
The error was caused by insufficient credentials to set
"trusted.overlay.redirect" xattr on link of a metacopy file.

Reproducer:

 echo Y > /sys/module/overlay/parameters/redirect_dir
 echo Y > /sys/module/overlay/parameters/metacopy
 cd /tmp
 mkdir l u w m
 chmod 777 l u
 touch l/foo
 ln l/foo l/link
 chmod 666 l/foo
 mount -t overlay none -olowerdir=l,upperdir=u,workdir=w m
 su fsgqa
 ln m/foo m/bar
 [   21.455823] overlayfs: failed to set redirect (-1)
 ln: failed to create hard link 'm/bar' => 'm/foo':\
     Invalid cross-device link

Reported-by: Theodore Y. Ts'o <tytso@mit.edu>
Reported-by: Maciej Zięba <maciekz82@gmail.com>
Fixes: 4120fe64 ("ovl: Set redirect on upper inode when it is linked")
Cc: <stable@vger.kernel.org> # v4.19
Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Acked-by: Vivek Goyal <vgoyal@redhat.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>