IB: Add ib_modify_qp_is_ok() library function

The in-kernel mthca driver contains a table of which attributes are
valid for each queue pair state transition.  It turns out that both
other IB drivers -- ipath and ehca -- which are being prepared for
merging have copied this table, errors and all.

To forestall this code duplication, move this table and the code to
check parameters against it into a midlayer library function,
ib_modify_qp_is_ok().

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