staging: erofs: move erofs_xattr_handlers to xattr.h

Let's move independent xattr-related stuffs to xattr.h.
No logic changes.

Reviewed-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Gao Xiang <gaoxiang25@huawei.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>