ceph: remove the incorrect Fw reference check when dirtying pages

[ Upstream commit c08dfb1b49492c09cf13838c71897493ea3b424e ]

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>
Signed-off-by: Sasha Levin <sashal@kernel.org>