include/linux/sched/signal.h: replace `tsk' with `task'

This file uses "task" 85 times and "tsk" 25 times.  It is better to be
consistent.

Link: http://lkml.kernel.org/r/20181129180547.15976-1-avagin@gmail.com
Signed-off-by: Andrei Vagin <avagin@gmail.com>
Reviewed-by: Andrew Morton <akpm@linux-foundation.org>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: "Eric W. Biederman" <ebiederm@xmission.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>