IB/core: Refactor to avoid unnecessary check on GID lookup miss

Currently on every gid entry comparison miss found variable is checked;
which is not needed as those two comparison fail already indicate that
GID is not found yet.
So refactor to avoid such check and copy the GID index when found.

Signed-off-by: Parav Pandit <parav@mellanox.com>
Reviewed-by: Daniel Jurgens <danielj@mellanox.com>
Signed-off-by: Leon Romanovsky <leon@kernel.org>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>