tracing: Add "within" filter for call-stack-based event filtering

Low-level kernel functions are called from many different paths.
When debugging, it is often useful to filter trace events to only
those occurring within a specific call chain.

Add a "within" filter predicate that tests whether a given function
appears in the current call stack at event time. The function name
is resolved to its address range via kallsyms during filter setup;
at runtime, stack_trace_save() captures the call stack and compares
each return address against the stored range.

Example:
  echo 'within == "vfs_read"' > events/sched/sched_switch/filter

Only "==" and "!=" operators are supported. The filter depends on
CONFIG_STACKTRACE.

Link: https://patch.msgid.link/20260821233050.335986786@kernel.org
Link: https://patch.msgid.link/20260608145556.94931-1-chenjun102@huawei.com
Signed-off-by: Chen Jun <chenjun102@huawei.com>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>

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