IB/core: Bound check alternate path port number

The alternate port number is used as an array index in the IB
security implementation, invalid values can result in a kernel panic.

Cc: <stable@vger.kernel.org> # v4.12
Fixes: d291f1a6 ("IB/core: Enforce PKey security on QPs")
Signed-off-by: Daniel Jurgens <danielj@mellanox.com>
Reviewed-by: Parav Pandit <parav@mellanox.com>
Signed-off-by: Leon Romanovsky <leon@kernel.org>
Signed-off-by: Doug Ledford <dledford@redhat.com>