ceph: fix cache revoke race

handle following sequence of events:

- non-auth MDS revokes Fc cap. queue invalidate work
- auth MDS issues Fc cap through request reply. i_rdcache_gen gets
  increased.
- invalidate work runs. it finds i_rdcache_revoking != i_rdcache_gen,
  so it does nothing.

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