ceph: don't zero i_wrbuffer_ref when reconnecting is denied

remove_session_caps_cb() does not truncate dirty data in page
cache, but zeros i_wrbuffer_ref/i_wrbuffer_ref_head. This will
result negtive i_wrbuffer_ref/i_wrbuffer_ref_head

Signed-off-by: Yan, Zheng <zyan@redhat.com>