IB/uverbs: Use inline data transfer for UHW_IN

IB/uverbs: Use inline data transfer for UHW_IN

The rule for the API is pointers less than 8 bytes are inlined into
the .data field of the attribute. Fix the creation of the driver udata
struct to follow this rule and point to the .data itself when the size
is less than 8 bytes.

Otherwise if the UHW struct is less than 8 bytes the driver will get
EFAULT during copy_from_user.

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