When can a struct buf's b_lblkno field by < 0 ?

[Alan Somers]

In struct buf, b_lblkno is documented as "Logical block number".  I
would expect that to always be nonnegative.  However, vtruncbuf loops
through a list of buffers, skipping those where "bp->b_lblkno > 0".
Maybe that's just an awkward way of writing "do something for the
buffer where b_lblkno == 0", but SVN archaeology suggests otherwise.
Before r112182, the code looked like this, implying that the b_lblkno
could actually be negative:

if ((bp->b_flags & B_DELWRI) && (bp->b_lblkno < 0)) {

Does anybody know under what circumstances that field might be
negative?  Also, was r112182 a correct change?  It appears to have
negated "<" and gotten ">", neglecting the "==" case.
https://svnweb.freebsd.org/base/head/sys/kern/vfs_subr.c?r1=112182&r2=112181&pathrev=112182

-Alan