signal: Rename group_exit_task group_exec_task

The only remaining user of group_exit_task is exec.  Rename the field
so that it is clear which part of the code uses it.

Update the comment above the definition of group_exec_task to document
how it is currently used.

Link: https://lkml.kernel.org/r/20211213225350.27481-7-ebiederm@xmission.com


Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>