module: split taint adding with info checking

check_modinfo() actually does two things:

 a) sanity checks, some of which are fatal, and so we
    prevent the user from completing trying to load a module
 b) taints the kernel

The taints are pretty heavy handed because we're tainting the kernel
*before* we ever even get to load the module into the modules linked
list. That is, it it can fail for other reasons later as we review the
module's structure.

But this commit makes no functional changes, it just makes the intent
clearer and splits the code up where needed to make that happen.

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