perf/ftrace: Check for rcu_is_watching() in callback function

If a ftrace callback requires "rcu_is_watching", then it adds the
FTRACE_OPS_FL_RCU flag and it will not be called if RCU is not "watching".
But this means that it will use a trampoline when called, and this slows
down the function tracing a tad. By checking rcu_is_watching() from within
the callback, it no longer needs the RCU flag set in the ftrace_ops and it
can be safely called directly.

Link: https://lkml.kernel.org/r/[email protected]
Link: https://lkml.kernel.org/r/[email protected]

Cc: Peter Zijlstra <[email protected]>
Cc: Ingo Molnar <[email protected]>
Cc: Josh  Poimboeuf <[email protected]>
Cc: Jiri Kosina <[email protected]>
Cc: Miroslav Benes <[email protected]>
Cc: Petr Mladek <[email protected]>
Cc: Masami Hiramatsu <[email protected]>
Cc: Andrew Morton <[email protected]>
Cc: Jiri Olsa <[email protected]>
Signed-off-by: Steven Rostedt (VMware) <[email protected]>

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