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.

Change-Id: I8b62edc032cb8a6a7be67fa57e84a55805dae743
Signed-off-by: Kees Cook <keescook@chromium.org>
Cc: Rabin Vincent <rabin@rab.in>
Acked-by: Nicolas Pitre <nico@linaro.org>
Git-commit: 99b4ac9afce4129323b5b4c7002a942a9489914c
Git-repo: git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git


[shashim@codeaurora.org: resolve trivial merge conflicts]
Signed-off-by: Shiraz Hashim <shashim@codeaurora.org>