[openib-general] OpenIB DAT_Return merging and kdat.h

[Caitlin Bestler]

Eliminating the DAT_RETURN detail information is a strange
choice, but certainly within the realm of options available
to any implementation. No DAPL consumer should rely on the
more detailed information. It is strange because it may
encourage people to code from uDAPL in order to get more
detailed diagnostic information while generally you would
expect a kernel client to have access to more detailed info.

However the proposal to merge some of the major error codes
would make it impossible for a wrapper include file (such as
kdat.h) to restore the full API for use by insmodded clients.

Given both of those factors, I believe it makes more sense
to preserve the full DAT_RETURN value (including detail error
data) as an out parameter. kdat.h could then simply turn it 
into the functional return for compatability clients and there
would be no loss of information. Either that or we could work
out on a case by case basis how kdat.h would be able to restore
the original error value. This may be possible with data in
the struct, since multiple outstanding errors on a single 
object would not seem to be likely (or perhaps more appropriately
worthy of providing detailed error diagnostics for, since it
probably represents a totally hosed application).