mds: fix session_info_t decoding

commit 0bcf2ac081 changes session_info_t's format, but there is
a typo in the code that decodes old format. We also need to
handle struct_v == 1, which had the same encoding but without
the size guards (which is all handled by DECODE_START_LEGACY_COMPAT).

Signed-off-by: Yan, Zheng <zheng.z.yan@intel.com>
Signed-off-by: Greg Farnum <greg@inktank.com>