Fix zfs send progress reporting

The progress of a send is supposed to be reported by zfs send -v, but
it is not. This works by creating a new user thread (with
pthread_create()) which does ZFS_IOC_SEND_PROGRESS ioctls to check how
much progress has been made. This IOCTL finds the specified send (since
there may be multiple concurrent sends in the system). The IOCTL also
checks that the specified send was started by the current process.

On Linux, different threads of the same process are represented as
different struct task_structs (and, confusingly, have different
PID's). To check if if two threads are in the same process, we need to
check if they have the same struct task_struct:group_leader.

We used to to this correctly, but it was inadvertently changed by
30af21b02569 (Redacted Send) to simply check if the current
struct task_struct is the one that started the send.

This commit changes the code back to checking if the send was started by
a struct task_struct with the same group_leader as the calling
thread.

Reviewed-by: Ryan Moeller <ryan@iXsystems.com>
Reviewed-by: Chris Wedgwood <cw@f00f.org>
Reviewed-by: Paul Dagnelie <pcd@delphix.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Matthew Ahrens <mahrens@delphix.com>
Closes #10215
Closes #10216