perf: Remove get_perf_callchain() init_nr argument

The 'init_nr' argument has double duty: it's used to initialize both the
number of contexts and the number of stack entries.  That's confusing
and the callers always pass zero anyway.  Hard code the zero.

Signed-off-by: Josh Poimboeuf <jpoimboe@kernel.org>
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: Namhyung Kim <Namhyung@kernel.org>
Acked-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/r/20250820180428.259565081@kernel.org