iotrace: fix behaviour when buffer is full

iotrace: fix behaviour when buffer is full

Don't continue updating the offset when buffer is full.
When the buffer size exhausts and there's no space left to write
warn the user and update only the needed size and not both the
offset and needed size.

Add needed buffer size information in the iotrace command.

Signed-off-by: Ramon Fried <ramon.fried@gmail.com>