vtruncbuf: add a comment explaining the purpose of the loop

vtruncbuf: improve the check for meta buffer

Revision e99215a614675 reorganized the code in vtruncbuf(), and moved
the logic to flush meta buffers into a dedicated loop. While doing it,
the condition was changed from bp->b_lblkno < 0 (to handle) into
bp->b_lblkno > 0 (to skip), which causes buffer at lblkno to needlessly
flush.