ceph: fix length validation in parse_reply_info()

"len" is read from network and thus needs validation.  Otherwise, given
a bogus "len" value, p+len could be an out-of-bounds pointer, which is
used in further parsing.

Signed-off-by: Xi Wang <xi.wang@gmail.com>
Signed-off-by: Sage Weil <sage@newdream.net>