IB/ipath: Normalize error return codes for posting work requests

The error codes for ib_post_send(), ib_post_recv(), and ib_post_srq_recv()
were inconsistent. Use EINVAL for too many SGEs and ENOMEM for too many
WRs.

Signed-off-by: Ralph Campbell <ralph.campbell@qlogic.com>
Signed-off-by: Roland Dreier <rolandd@cisco.com>