ceph: fix error handling in ceph_get_caps()

The function return 0 even when interrupted or try_get_cap_refs()
return error.

Fixes: 1199d7da ("ceph: simplify arguments and return semantics of try_get_cap_refs")
Signed-off-by: "Yan, Zheng" <zyan@redhat.com>
Reviewed-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>