dtrace_io.4: Use bio_length instead of bio_bcount in examplesHEADmain

Tracing bio_bcount makes little sense for some devices like for example
md(4), as it is set to "0" instead of to the actual I/O length.

markj@ suggested the following DTrace one-liner to identify some cases
where bio_length is set but bio_bcount is not:

    dtrace -n 'io:::start /args[0]->bio_length != args[0]->bio_bcount/{printf("%d %d", args[0]->bio_length, args[0]->bio_bcount); stack();}'

For future reference in the context of bio_length vs bio_bcount,
phk@ mentioned in the code review that:

> the original intent was to get rid of of bio_bcount

Reviewed by:	markj
MFC after:	2 weeks
Differential Revision:	https://reviews.freebsd.org/D53365

0 files changed, 0 insertions, 0 deletions