libceph: change ceph_con_in_msg_alloc() to take hdr

ceph_con_in_msg_alloc() is protocol independent, but con->in_hdr (and
struct ceph_msg_header in general) is msgr1 specific.  While the struct
is deeply ingrained inside and outside the messenger, con->in_hdr field
can be separated.

Signed-off-by: Ilya Dryomov <idryomov@gmail.com>