exit: change zap_other_threads() to count sub-threads

exit: change zap_other_threads() to count sub-threads

Change zap_other_threads() to return the number of other sub-threads found
on ->thread_group list.

Other changes are cosmetic:

	- change the code to use while_each_thread() helper

	- remove the obsolete comment about SIGKILL/SIGSTOP

Signed-off-by: Oleg Nesterov <[email protected]>
Acked-by: Roland McGrath <[email protected]>
Cc: Veaceslav Falico <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>