ktr: Use STAILQ_EMPTY_ATOMIC when checking for records in ktr_drain()

This is an unlocked check, and after commit 34740937f7a4 the debug
checks in STAILQ_EMPTY may spuriously fail here. In particular, the per
process queue is updated under the global ktrace mutex, not held in
ktr_drain(). If a record is enqueued concurrently, the recording thread
will schedule an AST to drain the queue again, so it should not be
possible for a race to leave records in the queue indefinitely.

Reviewed by: kib, olce
Reported by: syzbot+d67eddd8c4923ee28bb7@syzkaller.appspotmail.com
MFC after: 2 weeks
Fixes: 34740937f7a4 ("queue: New debug macros for STAILQ")
Differential Revision: https://reviews.freebsd.org/D48899