tracing/sched: Use __string() instead of fixed lengths for task->comm

The sched_switch and sched_waking events hardcoded the length of the comm
it recorded because these events were created before the dynamic strings
were implemented. Unfortunately, several other events copied this method.

As the size of the comm may change in the future, make the string dynamic.
The dynamic string requires a 4 byte meta data to hold the size and offset
of the string. The amount stored in the ring buffer will then be the
strlen(comm) + 5 (for the \n), and aligned to 4 bytes if there's no other
strings. This means that a task comm can have up to 10 characters before it
requires another 4 bytes in the ring buffer. Most tasks are usually less
than that, so this should not be a problem, and it also allows the name to
be extended over the TASK_COMM_LEN [1]

Note, sched_switch and the sched_waking trace events still hardcode the
length, as there is tooling that still requires that. An effort to update
the tooling will be made to allow this to change in the future.

[1] https://lore.kernel.org/all/20250507110444.963779-1-bhupesh@igalia.com/

Cc: Masami Hiramatsu <mhiramat@kernel.org>
Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: Bhupesh <bhupesh@igalia.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Juri Lelli <juri.lelli@redhat.com>
Link: https://lore.kernel.org/20250507133458.51bafd95@gandalf.local.home
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>

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