OSD: allow build_incremental_map_msg to fail on lookups

Since we're now building incremental map messages out-of-band with doing
other map updates now, we need to tolerate lookup failures at the bottom
end. Do so by returning a NULL message in that case.
Handle that in send_incremental_map by looping until we get a
message back -- if we fail on the first attempt, we'll get
the OSDSuperblock again and deal with it.

Signed-off-by: Greg Farnum <greg@inktank.com>