mds: Add config option for log segment size

mds: Add config option for log segment size

The mds log segment size is chosen from the
default layout object size (4MB).  Add a parameter
to the config to enable setting the log segment
size to an alternate value.

If the config option to set the journal log segment
size is specified, the log layout must be modified
both for the object size and the striping unit size.

Signed-off-by: Sam Lang <sam.lang@inktank.com>
Reviewed-by: Greg Farnum <greg@inktank.com>