arm64: use fixmap for text patching

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.

Reviewed-by: Kees Cook <[email protected]>
Reviewed-by: Mark Rutland <[email protected]>
Tested-by: Kees Cook <[email protected]>
Tested-by: Mark Rutland <[email protected]>
Signed-off-by: Laura Abbott <[email protected]>
Signed-off-by: Catalin Marinas <[email protected]>