BACKPORT: audit: consistently record PIDs with task_tgid_nr()

Unfortunately we record PIDs in audit records using a variety of
methods despite the correct way being the use of task_tgid_nr().
This patch converts all of these callers, except for the case of
AUDIT_SET in audit_receive_msg() (see the comment in the code).

Reported-by: Jeff Vander Stoep <jeffv@google.com>
Signed-off-by: Paul Moore <paul@paul-moore.com>

Bug: 28952093

(cherry picked from commit fa2bea2f5cca5b8d4a3e5520d2e8c0ede67ac108)
Signed-off-by: Jeff Vander Stoep <jeffv@google.com>
Change-Id: I36508a25c957f5108299e68a3b0f627c94eb27eb