support forced patching

Some functions in the kernel are always on the stack of some thread
in the system.  Attempts to patch these function will currently always
fail the activeness safety check.

However, through human inspection, it can be determined that, for a
particular function, consistency is maintained even if the old and new
versions of the function run concurrently.

This commit introduces a KPATCH_FORCE_UNSAFE() macro to define patched
functions that such be exempted from the activeness safety check.

Signed-off-by: Seth Jennings <sjenning@redhat.com>