gve: Add support for 4k RX Buffers when using DQO queue formats

This change adds support for using 4K RX Buffers when using DQO queue
formats when a boot-time tunable flag is set to true by the user.
When this flag is enabled, the driver will use 4K RX Buffer size either
when HW LRO is enabled or mtu > 2048.

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

Reviewed by: markj, ziaee
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D50786