osd: OSD: better explanation on 'max_count' calculation for 'osd bench'

'max_count' is the maximum number of bytes that we are to allow for an
'osd bench' command.  This value is a hard-cap that takes into account
a predefined throughput, the 'osd bench' duration and, for a rather large
block size, can be taken as the amount of bytes that we would ever be
able to write in that period of time.

The explanation wasn't appropriate enough, hence this patch, which
hopefully will avoid confusion in the future.

Signed-off-by: Joao Eduardo Luis <joao.luis@inktank.com>
qa/workunits: cephtool: fix 'osd bench' test

Commit 7dc93a9651f602d9c46311524fc6b54c2f1ac595 fixed an incorrect
behavior with the OSD's 'osd bench' value hard-caps.  The test wasn't
appropriately modified unfortunately.

Signed-off-by: Joao Eduardo Luis <joao.luis@inktank.com>