powerpc/code-patching: Pre-map patch area

JIRA: https://issues.redhat.com/browse/RHEL-8585

Backport of three commits to resolve a sleeping while atomic
issue in powerpc. We need to disable interrupts as we are attempting to
patch kernel code, but we also need to allocate a page. Upstream
resolves this by pre-allocating pages on boot before this situation
could occur.

Signed-off-by: Audra Mitchell audra@redhat.com