mds: check cap ID when handling cap export message

handle following sequence of events:
- mds0 exports an inode to mds1. client receives the cap import
  message from mds1. caps from mds0 are removed while handling
  the cap import message.
- mds1 exports an inode to mds0. client receives the cap export
  message from mds1. handle_cap_export() adds placeholder caps
  for mds0
- client receives the first cap export message (for exporting
  inode from mds0 to mds1)

Signed-off-by: Yan, Zheng <zheng.z.yan@intel.com>