Free some space in struct ctl_io_hdr for better use.

• Collapse original_sc and serializing_sc fields into one, since they

are never used simultanously, we have only one local I/O and one remote.

• Move remote_sglist and local_sglist fields into CTL_PRIV_BACKEND,

since they are used only on Originating SC in XFER mode, where requests
don't ever reach backends, so we can reuse backend's private storage.

MFC after: 2 weeks
Sponsored by: iXsystems, Inc.