ceph: messenger: reset connection kvec caller

Reset a connection's kvec fields in the caller rather than in
prepare_write_connect().   This ends up repeating a few lines of
code but it's improving the separation between distinct operations
on the connection, which we can take advantage of later.

Signed-off-by: Alex Elder <elder@inktank.com>
Reviewed-by: Sage Weil <sage@inktank.com>