IB/rxe: Hold refs when running tasklets

It might be possible for all of a QP's references to be dropped
while one of that QP's tasklets is running.

For example, the completer might run during QP destroy.
If qp->valid is false, it will drop all of the packets on
the resp_pkts list, potentially removing the last reference.
Then it tries to advance the SQ consumer pointer. If the
SQ's buffer has already been destroyed, the system will
panic.

To be safe, hold a reference on the QP for the duration
of each tasklet.

Signed-off-by: Andrew Boyer <andrew.boyer@dell.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>