UPSTREAM: arm: fixmap: implement __set_fixmap()

This is used from set_fixmap() and clear_fixmap() via asm-generic/fixmap.h.
Also makes sure that the fixmap allocation fits into the expected range.

Based on patch by Rabin Vincent.

Signed-off-by: Kees Cook <keescook@chromium.org>
Cc: Rabin Vincent <rabin@rab.in>
Acked-by: Nicolas Pitre <nico@linaro.org>

(cherry picked from commit 99b4ac9afce4129323b5b4c7002a942a9489914c)
Signed-off-by: Kees Cook <keescook@google.com>
Change-Id: I75289a69240a5e36902e66c75dba0bef1b2ec05d