Luster has a container_of0() function which is similar to
container_of() but passes an IS_ERR_OR_NULL() pointer through
unchanged.
This could be generally useful: bcache at last has a similar function.

Naming is hard, but the precedent set by hlist_entry_safe() suggests
a _safe suffix might be most consistent.

So add container_of_safe() to kernel.h, and replace all occurrences of
container_of0() with one of
  - list_first_entry, list_next_entry, when that is a better fit,
  - container_of(), when the pointer is used as a validpointer in
    surrounding code,
  - container_of_safe() when there is no obviously better alternative.

Signed-off-by: NeilBrown <neilb@suse.com>
Reviewed-by: James Simmons <jsimmons@infradead.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>