mds: fix byte_range_t ctor

I do not think we saw any bugs from this, but anything that involved
capability issues on restart or migrate might have been caused by
this.

Signed-off-by: Sage Weil <sage@inktank.com>
Signed-off-by: Greg Farnum <greg@inktank.com>
mds: fix default_file_layout constructor

Signed-off-by: Greg Farnum <greg@inktank.com>
Reviewed-by: Sage Weil <sage@inktank.com>