ovl: Switch to generic_getxattr

ovl: Switch to generic_getxattr

Now that overlayfs has xattr handlers for iop->{set,remove}xattr, use
those same handlers for iop->getxattr as well.

Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>
Signed-off-by: Shan Hai <shan.hai@oracle.com>
Orabug: 26401569

(backport upstream commit 0eb45fc3bb7a2cf9c9c93d9e95986a841e5f4625)

Signed-off-by: Shan Hai <shan.hai@oracle.com>
Reviewed-by: Somasundaram Krishnasamy <somasundaram.krishnasamy@oracle.com>