Start a new header, errcode.h, for bcachefs-private error codes - more
error codes will be converted later.

This patch just converts bucket_alloc_ret so that they can be mixed with
standard error codes and passed as ERR_PTR errors - the ec.c code was
doing this already, but incorrectly.
Signed-off-by: Kent Overstreet <kent.overstreet@gmail.com>