osd: OSD: limit the value of 'size' and 'count' on 'osd bench'

Otherwise, a high enough 'count' value will trigger all sorts of timeouts
on the OSD; a low enough 'size' value will have the same effect for a
high enough value of 'count' (even the default value may have ill effects
on the osd's behaviour).  Limiting these values do not fix how 'osd bench'
should behave, but avoid someone from inadvertently bork an OSD.

Three options have been added and the user may adjust them if he so
desires to play with the OSD's fate:

 - 'osd_bench_small_size_max_iops' [default: 100] defines the amount of
   expected IOPS for a small block size (i.e., <1MB).
 - 'osd_bench_large_size_max_throughput' [default: 100<<20] defines
   the expected throughput in B/s.  We assume 100MB/s.
 - 'osd_bench_max_block_size' [default: 64 << 20] caps the block size
   allowed.  We have defined 64 MB.

These options attempt to avoid combinations of dangerous parameters.  For
instance, we limit the block size to 64 MB (by default) so that there is
no temptation to specify a large enough block size, along with a very small
'count', such that the end result is similar to specifying a big count with
a sane block size.

Fixes: 7248

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