mds: share max size to client who is allowed for WR cap

WR cap is allowed for the loner client when filelock is in excl->mix
state. MDS should share max size with the loner client in this case.
Otherwise the client may wait for the max size forever.

Fixes: #8092
Signed-off-by: Yan, Zheng <zheng.z.yan@intel.com>
mds: fix cap revoke confirmation

when the _revokes list is emptied, it doesn't mean that client has
released the revoking caps. It's possible that client was flusing
dirty metadata.

Signed-off-by: Yan, Zheng <zheng.z.yan@intel.com>
Merge pull request #1676 from ceph/wip-8092

Wip 8092

Reviewed-by: Sage Weil <sage@inktank.com>