modpost: introduce get_basename() helper

The logic to retrieve the basename appears multiple times.
Factor out the common pattern into a helper function.

I copied kbasename() from include/linux/string.h and renamed it
to get_basename().

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Reviewed-by: Nicolas Schier <nicolas@fjasle.eu>