IB/mlx4: Use correct subnet-prefix in QP1 mads under SR-IOV

When sending QP1 MAD packets which use a GRH, the source GID
(which consists of the 64-bit subnet prefix, and the 64 bit port GUID)
must be included in the packet GRH.

For SR-IOV, a GID cache is used, since the source GID needs to be the
slave's source GID, and not the Hypervisor's GID. This cache also
included a subnet_prefix. Unfortunately, the subnet_prefix field in
the cache was never initialized (to the default subnet prefix 0xfe80::0).
As a result, this field remained all zeroes.  Therefore, when SR-IOV
was active, all QP1 packets which included a GRH had a source GID
subnet prefix of all-zeroes.

However, the subnet-prefix should initially be 0xfe80::0 (the default
subnet prefix). In addition, if OpenSM modifies a port's subnet prefix,
the new subnet prefix must be used in the GRH when sending QP1 packets.
To fix this we now initialize the subnet prefix in the SR-IOV GID cache
to the default subnet prefix. We update the cached value if/when OpenSM
modifies the port's subnet prefix. We take this cached value when sending
QP1 packets when SR-IOV is active.

Note that the value is stored as an atomic64. This eliminates any need
for locking when the subnet prefix is being updated.

Note also that we depend on the FW generating the "port management change"
event for tracking subnet-prefix changes performed by OpenSM. If running
early FW (before 2.9.4630), subnet prefix changes will not be tracked (but
the default subnet prefix still will be stored in the cache; therefore
users who do not modify the subnet prefix will not have a problem).
IF there is a need for such tracking also for early FW, we will add that
capability in a subsequent patch.

Fixes: 1ffeb2eb

 ("IB/mlx4: SR-IOV IB context objects and proxy/tunnel SQP support")
Signed-off-by: Jack Morgenstein <jackm@dev.mellanox.co.il>
Signed-off-by: Leon Romanovsky <leon@kernel.org>
Signed-off-by: Doug Ledford <dledford@redhat.com>