xattr: constify value argument in vfs_setxattr()

Now that we don't perform translations directly in vfs_setxattr()
anymore we can constify the @value argument in vfs_setxattr(). This also
allows us to remove the hack to cast from a const in ovl_do_setxattr().

Signed-off-by: Christian Brauner (Microsoft) <brauner@kernel.org>
Reviewed-by: Seth Forshee (DigitalOcean) <sforshee@kernel.org>