module: add debugging auto-load duplicate module support

This adds debugging support to the kernel module auto-loader to
easily detect and deal with duplicate module requests. To aid
with possible bootup failure issues it will supress the waste
in virtual memory when races happen before userspace loads a
module and the kernel is still issuing requests for the same
module.

Folks debugging virtual memory abuse on bootup can and should
enable this to see what WARN()s come on, to see if module
auto-loading is to blame for their woes.

Signed-off-by: Luis Chamberlain <mcgrof@kernel.org>