arm64: use fixmap for text patching

When kernel text is marked as read only, it cannot be modified directly.
Use a fixmap to modify the text instead in a similar manner to
x86 and arm.

Change-Id: I466a1d3cdab4f5cbb940710f613526cf31cf9387
Reviewed-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Mark Rutland <mark.rutland@arm.com>
Tested-by: Kees Cook <keescook@chromium.org>
Tested-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Laura Abbott <lauraa@codeaurora.org>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
Git-commit: 2f896d5866107e2926dcdec34a7d40bc56dd2951)
Git-repo: git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git


[rvaswani@codeaurora.org: fixed merge conflicts]
Signed-off-by: Rohit Vaswani <rvaswani@codeaurora.org>