libceph: change ceph_con_in_msg_alloc convention to be less weird

libceph: change ceph_con_in_msg_alloc convention to be less weird

(cherry picked from commit 4740a623d20c51d167da7f752b63e2b8714b2543)

This function's calling convention is very limiting.  In particular,
we can't return any error other than ENOMEM (and only implicitly),
which is a problem (see next patch).

Instead, return an normal 0 or error code, and make the skip a pointer
output parameter.  Drop the useless in_hdr argument (we have the con
pointer).

Signed-off-by: Sage Weil <sage@inktank.com>
Reviewed-by: Alex Elder <elder@inktank.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>