libceph: make decode_pool() more resilient against corrupted osdmaps

In the Linux kernel, the following vulnerability has been resolved:

libceph: make decode_pool() more resilient against corrupted osdmaps

If the osdmap is (maliciously) corrupted such that the encoded length of ceph_pg_pool envelope is less than what is expected for a particular encoding version, out-of-bounds reads may ensue because the only bounds check that is there is based on that length value.

This patch adds explicit bounds checks for each field that is decoded or skipped.