Fix LOCAL_PEERCRED in 32-bit compat mode

[asomers]

Summary

Previously the cr_pid field would be incorrectly copied to userland, due
to a size mismatch between the structure as defined in 32-bit vs 64-bit
builds. Fix it by converting the structure before copying it to
userland.

PR: 294833
Sponsored by: ConnectWise
MFC after: 1 week

Test Plan

Used the test program in Bugzilla

[emaste]

sys/kern/uipc_usrreq.c

2760

[asomers]

• Respond to emaste's style comment.

[emaste]

Not just style, we don't want to leak kernel stack in struct gaps

[emaste]

LGTM, feel free to give other reviewers some time.

[asomers]

In D56675#1298767, @emaste wrote:

LGTM, feel free to give other reviewers some time.

Thanks Ed. I'll wait a little while before committing.