virtio_ring: Avoid using inline for small functions

According to kernel coding style [1], defining inline functions is not
necessary and beneficial for simple functions. Hence clean up the code
by removing the inline keyword.

It is verified with GCC 12.2.0, the generated code with/without inline
is same. Additionally tested with pktgen and iperf, and verified the
result, the pps test results are the same in the cases of with/without
inline.

Iperf and pps of pktgen for virtio-net didn't change before and after
the change.

[1]
https://www.kernel.org/doc/html/v6.2-rc3/process/coding-style.html#the-inline-disease



Signed-off-by: Feng Liu <feliu@nvidia.com>
Reviewed-by: Jiri Pirko <jiri@nvidia.com>
Reviewed-by: Parav Pandit <parav@nvidia.com>
Reviewed-by: Gavin Li <gavinl@nvidia.com>
Reviewed-by: Bodong Wang <bodong@nvidia.com>
Reviewed-by: David Edmondson <david.edmondson@oracle.com>
Message-Id: <20230310053428.3376-3-feliu@nvidia.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>