mon: include 'max avail' in df output

mon: include 'max avail' in df output

Include an estimate of the maximum writeable space for each pool.  Note
that this value is a conservative estimate for that pool based on the
most-full OSD.  It is also potentially misleading as it is the available
space if *all* new data were written to this pool; one cannot (generally)
add up the available space for all pools.

Signed-off-by: Sage Weil <sage@inktank.com>