kmod/core: taint kernel with TAINT_USER

For now, taint with TAINT_USER when loading a patch module so that the
user can always detect when a kpatch module has been previously loaded.
Eventually we will want a dedicated TAINT_KPATCH flag in the kernel.
Merge pull request #124 from jpoimboe/taint

kmod/core: taint kernel with TAINT_USER