gve: Use load-acquire to fetch generation bits

When running the driver using the DQO queue format, we must load the
generation bit and check it before possibly reading the rest of the
descriptor's fields.

Previously, we guarded against reordering of reads using an explicit
thread fence. This commit changes the thread fence to a load with
acquire semantics. Because the tx and rx generation fields are in a
bitfield, we cannot explicitly address them in an atomic load. Instead
we load the respective containing bytes in the descriptor and mask them
appropriately.

Signed-off-by: Jasper Tran O'Leary <jtranoleary@google.com>

Reviewed by: markj
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D50384

(cherry picked from commit b044f125373e05c20ca16edfd29cccad1f4025bd)