PTR_RET is now PTR_ERR_OR_ZERO

True, it's often used in return statements, but after much bikeshedding
it's probably better to have an explicit name.

(I tried just putting the IS_ERR check inside PTR_ERR itself and gcc
usually generated no more code.  But that clashes current expectations
of how PTR_ERR behaves, so having a separate function is better).

Suggested-by: Julia Lawall <julia.lawall@lip6.fr>
Suggested-by: "Michael S. Tsirkin" <mst@redhat.com>
Cc: Julia Lawall <julia.lawall@lip6.fr>
Cc: "Michael S. Tsirkin" <mst@redhat.com>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>