libibmad: encode SM_Key in SA MAD headers

mad_encode() always encodes rpc->mkey in IB_MAD_MKEY_F. For an SA MAD,
that location is part of the RMPP header; the SA SM_Key is stored later
in IB_SA_MKEY_F.

This was harmless for existing SA users because rpc->mkey was always
zero. Code requiring a nonzero key instead had to build the packet and
patch IB_SA_MKEY_F afterward.

Encode rpc->mkey according to the management class, using
IB_SA_MKEY_F for SA MADs and retaining IB_MAD_MKEY_F for other
classes. Reusing rpc->mkey avoids extending the public ib_rpc_t
structure and treats it as the generic management-key value for the
selected class.

Update sa_query() to set rpc.mkey before building the packet instead of
patching the encoded SA MAD directly.

Signed-off-by: Jonathan Süssemilch Poulain <jpoulain@coreweave.com>