pvrdma: wean code off pvrdma_ring.h kernel header

pvrdma: wean code off pvrdma_ring.h kernel header

The pvrdma code relies on the pvrdma_ring.h kernel header for some
basic ring buffer handling. The content of that header isn't very
exciting, but contains some (q)atomic_*() invocations that (a)
cause manual massaging when doing a headers update, and (b) are
an indication that we probably should not be importing that header
at all.

Let's reimplement the ring buffer handling directly in the pvrdma
code instead. This arguably also improves readability of the code.

Importing the header can now be dropped.

Signed-off-by: Cornelia Huck <cohuck@redhat.com>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Yuval Shaia <yuval.shaia.ml@gmail.com>
Tested-by: Yuval Shaia <yuval.shaia.ml@gmail.com>
Signed-off-by: Jason Wang <jasowang@redhat.com>