Description: tty: Fix ->pgrp locking in tiocspgrp()
References:
 https://bugs.chromium.org/p/project-zero/issues/detail?id=2125
 https://googleprojectzero.blogspot.com/2021/10/how-simple-linux-kernel-memory.html
Notes:
 carnil> Fixed as well in 5.9.14.
Bugs:
upstream: released (5.10-rc7) [54ffccbf053b5b6ca4f6e45094b942fab92a25fc]
5.10-upstream-stable: N/A "Fixed before branch point"
4.19-upstream-stable: released (4.19.163) [13f10a78097df2f14d4e1fd390dbaa3e28502ca7]
4.9-upstream-stable: released (4.9.248) [742f3062298ac1ae1d28de31b1f946f93db1eba1]
sid: released (5.9.15-1)
4.19-buster-security: released (4.19.171-1)
4.9-stretch-security: released (4.9.258-1)