gve: Add feature to change TX/RX ring size

This change introduces new sysctl handlers that allow the user to change
RX/TX ring sizes. As before, the default ring sizes will come from the
device (usually 1024). We also get the max/min limits from the device.
In the case min values are not provided we have statically defined
constants for the min values. Additionally, if the modify ring option is
not enabled on the device, changing ring sizes via sysctl will not be
possible. When changing ring sizes, the interface turns down
momentarily while allocating/freeing resources as necessary.

Signed-off-by: Vee Agarwal <veethebee@google.com>

Reviewed by: markj
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D49428