container_of: Add container_of_or_null()

container_of: Add container_of_or_null()

It's quite common to have a pointer which could be either a pointer to a
structure member, or NULL.  This needs special casing with container_of(),
or it will convert NULL into something strange.

This patch adds container_of_or_null(), which will return NULL if passed
(an appropriately typed) NULL, or the containining structure as
container_of() otherwise.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>