convert *_new() functions to return the new object directly

convert *_new() functions to return the new object directly

Al Viro writes:

BTW, the calling conventions for your foo_new() are annoying - instead of
"return -E... or 0, storing the reference to new object in var parameter
passed as the last argument", could you please just return ERR_PTR(-E...)
on error, a pointer to new object on success and to hell with those
struct foo **foo in the argument lists?

Signed-off-by: Daniel Mack <daniel@zonque.org>