modpost: check forbidden MODULE_IMPORT_NS("module:") at compile time

modpost: check forbidden MODULE_IMPORT_NS("module:") at compile time

Explicitly adding MODULE_IMPORT_NS("module:...") is not allowed.

Currently, this is only checked at run time. That is, when such a
module is loaded, an error message like the following is shown:

  foo: module tries to import module namespace: module:bar

Obviously, checking this at compile time improves usability.

In such a case, modpost will report the following error at compile time:

  ERROR: modpost: foo: explicitly importing namespace "module:bar" is not allowed.

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