ceph: defer clearing the CEPH_I_FLUSH_SNAPS flag

ceph: defer clearing the CEPH_I_FLUSH_SNAPS flag

Clear the flag just after the capsnap request being sent out. Else the
ceph_check_caps() will race with it and send the cap update request
just before this capsnap request. Which will cause the cap update request
to miss setting the CEPH_CLIENT_CAPS_PENDING_CAPSNAP flag and finally
the mds will drop the capsnap request to floor.

URL: https://tracker.ceph.com/issues/64209
URL: https://tracker.ceph.com/issues/65705
Signed-off-by: Xiubo Li <xiubli@redhat.com>