ktrace: Use STAILQ_EMPTY_ATOMIC when checking for records in userret()

As in commit 36631977d8c9, this check is unlocked and may trigger
spurious assertion failures. Use STAILQ_EMPTY_ATOMIC() here as well.
Fix nearby whitespace.

Reported by: syzkaller
Reviewed by: olce
Fixes: 34740937f7a4 ("queue: New debug macros for STAILQ")
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D49441