Fix bug in vtruncbuf introduced by r346162

r346162 factored out v_inval_buf_range from vtruncbuf, but it made an error
in the interface between the two. The result was a failure to remove
buffers past the first. Surprisingly, I couldn't reproduce the failure with
file systems other than fuse.

Also, modify fusefs's truncate_discards_cached_data test to catch this bug.

PR: 346162
Sponsored by: The FreeBSD Foundation