modpost: rework and consolidate logging interface

Rework modpost's logging interface by consolidating merror(), warn(), and
fatal() to use a single function, modpost_log(). Introduce different
logging levels (WARN, ERROR, FATAL) as well. The purpose of this cleanup is
to reduce code duplication when deciding whether or not to warn or error
out based on a condition.

Signed-off-by: Jessica Yu <jeyu@kernel.org>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>