libceph: fix crush_decode() call site in osdmap_decode()

The size of the memory area feeded to crush_decode() should be limited
not only by osdmap end, but also by the crush map length.  Also, drop
unnecessary dout() (dout() in crush_decode() conveys the same info) and
step past crush map only if it is decoded successfully.

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