The existing error handling scheme requires resetting err to -EINVAL
prior to calling any ceph_decode_* macro. This is ugly and fragile,
and there already are a few places where we would return 0 on error,
due to a missing reset. Fix this by adding a special e_inval label to
be used by all ceph_decode_* macros.
Signed-off-by: Ilya Dryomov <ilya.dryomov@inktank.com> Reviewed-by: Alex Elder <elder@linaro.org>