ovl: take mnt_want_write() for removing impure xattr

commit a5a927a7c82e28ea76599dee4019c41e372c911f upstream.

The optimization in ovl_cache_get_impure() that tries to remove an
unneeded "impure" xattr needs to take mnt_want_write() on upper fs.

Fixes: 4edb83bb ("ovl: constant d_ino for non-merge dirs")
Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>