staging: kernel.h: Prevent macro expantion bug in container_of_safe()

There aren't many users of this so it doesn't cause a problem, but we
obviously want to use "__mptr" here instead of "ptr" to prevent the
parameter from being executed twice.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: NeilBrown <neilb@suse.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>