client: fix I_COMPLETE_ORDERED checking

Current code marks a directory inode as complete and ordered when readdir
finishes, but it does not check if the directory was modified in the middle
of readdir. This is wrong, directory inode should not be marked as ordered
if it was modified during readddir

The fix is introduce a new counter to the inode data struct, we increase
the counter each time the directory is modified. When readdir finishes, we
check the counter to decide if the directory should be marked as ordered.

Fixes: #9894
Signed-off-by: Yan, Zheng <zyan@redhat.com>
Merge pull request #2813 from ceph/wip-9894

client: fix I_COMPLETE_ORDERED checking

Reviewed-by: Sage Weil <sage@redhat.com>
Reviewed-by: Greg Farnum <gfarnum@redhat.com>