Fix a boot crash on a c8000 machine as reported by Dave.  Basically it changes
patch_map() to return an alias mapping to the to-be-patched code in order to
prevent writing to write-protected memory.

Signed-off-by: Helge Deller <deller@gmx.de>
Suggested-by: John David Anglin <dave.anglin@bell.net>
Cc: stable@vger.kernel.org   # v5.2+
Link: https://lore.kernel.org/all/e8ec39e8-25f8-e6b4-b7ed-4cb23efc756e@bell.net/