libceph: dump osdmap and enhance output on decode errors

Dump osdmap in hex on both full and incremental decode errors, to make
it easier to match the contents with error offset.  dout() map epoch
and max_osd value on success.

Signed-off-by: Ilya Dryomov <ilya.dryomov@inktank.com>
Reviewed-by: Alex Elder <elder@linaro.org>