ktrace: Remove vrele() at the end of ktr_writerequest()

As of commit fc369a353 we no longer ref the vnode when writing a record.
Drop the corresponding vrele() call in the error case.

Fixes: fc369a353 ("ktrace: fix a race between writes and close")
Reported by: syzbot+9b96ea7a5ff8917d3fe4@syzkaller.appspotmail.com
Reported by: syzbot+6120ebbb354cd52e5107@syzkaller.appspotmail.com
Reviewed by: kib
MFC after: 6 days
Differential Revision: https://reviews.freebsd.org/D30404