staging/rdma/hfi1: Add a credit push on diagpkt allocate fail

When sending a diagnostic packet, if the send context does not
have enough room, force a credit return and try again.

Reviewed-by: Dennis Dalessandro <dennis.dalessandro@intel.com>
Signed-off-by: Dean Luick <dean.luick@intel.com>
Signed-off-by: Jubin John <jubin.john@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>