arm/patch: Fix !MMU compile

Now that patch.o is unconditionally selected for ftrace, it can also
get compiled for !MMU kernels. These (obviously) lack
{set,clear}_fixmap() support.

Also remove the superfluous __acquire/__release nonsense.

Fixes: 42e51f187f86 ("arm/ftrace: Use __patch_text()")
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>