ceph: define new argument structure for send_cap_msg

When we get to this many arguments, it's hard to work with positional
parameters. send_cap_msg is already at 25 arguments, with more needed.

Define a new args structure and pass a pointer to it to send_cap_msg.
Eventually it might make sense to embed one of these inside
ceph_cap_snap instead of tracking individual fields.

Signed-off-by: Jeff Layton <jlayton@redhat.com>
Reviewed-by: Yan, Zheng <zyan@redhat.com>