ceph: remove unneeded test in try_flush_caps

cap->session is always non-NULL, so we can just do a single test for
equality w/o testing explicitly for a NULL pointer.

Signed-off-by: Jeff Layton <jlayton@kernel.org>
Reviewed-by: "Yan, Zheng" <zyan@redhat.com>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>