scripts/gdb: add helper and convenience function to look up tasks

scripts/gdb: add helper and convenience function to look up tasks

Add the helper task_by_pid that can look up a task by its PID.  Also
export it as a convenience function.

Signed-off-by: Jan Kiszka <[email protected]>
Cc: Thomas Gleixner <[email protected]>
Cc: Jason Wessel <[email protected]>
Cc: Andi Kleen <[email protected]>
Cc: Ben Widawsky <[email protected]>
Cc: Borislav Petkov <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>