IB/mlx4: Fix corruption of RoCEv2 IPv4 GIDs

When using IPv4 addresses in RoCEv2, the GID format for the mapped
IPv4 address should be: ::ffff:<4-byte IPv4 address>.

In the cited commit, IPv4 mapped IPV6 addresses had the 3 upper dwords
zeroed out by memset, which resulted in deleting the ffff field.

However, since procedure ipv6_addr_v4mapped() already verifies that the
gid has format ::ffff:<ipv4 address>, no change is needed for the gid,
and the memset can simply be removed.

Fixes: 7e57b85c

 ("IB/mlx4: Add support for setting RoCEv2 gids in hardware")
Reviewed-by: Moni Shoua <monis@mellanox.com>
Signed-off-by: Jack Morgenstein <jackm@dev.mellanox.co.il>
Signed-off-by: Leon Romanovsky <leon@kernel.org>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>