function_graph: Use static_call and branch to optimize entry function

In most cases function graph is used by a single user. Instead of calling
a loop to call function graph callbacks in this case, call the function
entry callback directly.

Add a static_key that will be used to set the function graph logic to
either do the loop (when more than one callback is registered) or to call
the callback directly if there is only one registered callback.

Link: https://lore.kernel.org/linux-trace-kernel/[email protected]

Cc: Masami Hiramatsu <[email protected]>
Cc: Mark Rutland <[email protected]>
Cc: Mathieu Desnoyers <[email protected]>
Cc: Andrew Morton <[email protected]>
Cc: Alexei Starovoitov <[email protected]>
Cc: Florent Revest <[email protected]>
Cc: Martin KaFai Lau <[email protected]>
Cc: bpf <[email protected]>
Cc: Sven Schnelle <[email protected]>
Cc: Alexei Starovoitov <[email protected]>
Cc: Jiri Olsa <[email protected]>
Cc: Arnaldo Carvalho de Melo <[email protected]>
Cc: Daniel Borkmann <[email protected]>
Cc: Alan Maguire <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Thomas Gleixner <[email protected]>
Cc: Guo Ren <[email protected]>
Reviewed-by: Masami Hiramatsu (Google) <[email protected]>
Signed-off-by: Steven Rostedt (Google) <[email protected]>

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