ceph: don't WARN if we're forcibly removing the session caps

For example in the case of a forced umount, we'll remove all the session
caps even if they are dirty. Move the warning to a wrapper function and
make most of the callers use it. Call the core function when removing
caps due to a forced umount.

Signed-off-by: Xiubo Li <xiubli@redhat.com>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>