osd: add sanity check/warning on a few key configs

Warn when certain config values are set to bad values.

Backport: firefly, dumpling Signed-off-by: Sage Weil <sage@inktank.com>
On Sun, 15 Jun 2014, Loic Dachary wrote:
> I guess nobody will try to set osd_map_cache_size to osd_map_max_advance + 1
> and be confused about the message.

Probably someone will, but it is better than worrying about an 
off-by-one in the complex interaction between the two values and 
having the cache be defeated.  Could just put a +2 in the warning, I 
guess?  :/

s