coredump: change zap_threads() and zap_process() to use for_each_thread()

Change zap_threads() paths to use for_each_thread() rather than
while_each_thread().

While at it, change zap_threads() to avoid the nested if's to make the
code more readable and lessen the indentation.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Cc: David Rientjes <rientjes@google.com>
Cc: Kyle Walker <kwalker@redhat.com>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: Stanislav Kozina <skozina@redhat.com>
Cc: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>