IB/uverbs: Fix possible oops with duplicate ioctl attributes

If the same attribute is listed twice by the user in the ioctl attribute
list then error unwind can cause the kernel to deref garbage.

This happens when an object with WRITE access is sent twice. The second
parse properly fails but corrupts the state required for the error unwind
it triggers.

Fixing this by making duplicates in the attribute list invalid. This is
not something we need to support.

The ioctl interface is currently recommended to be disabled in kConfig.

Signed-off-by: Matan Barak <matanb@mellanox.com>
Signed-off-by: Leon Romanovsky <leon@kernel.org>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>