ceph: remove the incorrect Fw reference check when dirtying pages

When doing the direct-io reads it will also try to mark pages dirty,
but for the read path it won't hold the Fw caps and there is case
will it get the Fw reference.

Fixes: 5dda377c ("ceph: set i_head_snapc when getting CEPH_CAP_FILE_WR reference")
Signed-off-by: Xiubo Li <xiubli@redhat.com>
Reviewed-by: Patrick Donnelly <pdonnell@redhat.com>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>