IB/mlx4: Fix memory leak in add_gid error flow

In procedure mlx4_ib_add_gid(), if the driver is unable to update the FW
gid table, there is a memory leak in the driver's copy of the gid table:
the gid entry's context buffer is not freed.

If such an error occurs, free the entry's context buffer, and mark the
entry as available (by setting its context pointer to NULL).

Fixes: e26be1bf

 ("IB/mlx4: Implement ib_device callbacks")
Link: https://lore.kernel.org/r/20200115085050.73746-1-leon@kernel.org
Signed-off-by: Jack Morgenstein <jackm@dev.mellanox.co.il>
Reviewed-by: Parav Pandit <parav@mellanox.com>
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>