[IB] simplify mad_rmpp.c:alloc_response_msg()

Change alloc_response_msg() in mad_rmpp.c to return the struct
it allocates directly (or an error code a la ERR_PTR), rather than
returning a status and passing the struct back in a pointer param.
This simplifies the code and gets rid of warnings like

    drivers/infiniband/core/mad_rmpp.c: In function nack_recv:
    drivers/infiniband/core/mad_rmpp.c:192: warning: msg may be used uninitialized in this function

with newer versions of gcc.

Signed-off-by: Roland Dreier <rolandd@cisco.com>