static_call: Add some validation

Verify the text we're about to change is as we expect it to be.

Requested-by: Steven Rostedt <rostedt@goodmis.org>

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Link: https://lore.kernel.org/r/20200818135805.161974981@infradead.org