kbuild: re-enable KCSAN for autogenerated *.mod.c intermediaries

This reverts commit 54babdc0 ("kbuild: Disable KCSAN for
autogenerated *.mod.c intermediaries").

Now that objtool is enabled for *.mod.c, there is no need to filter
out CFLAGS_KCSAN.

I no longer see "Unpatched return thunk in use. This should not happen!"
error with KCSAN when loading a module.

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