IB/uverbs: Always use the attribute size provided by the user

This fixes several bugs around the copy_to/from user path:
 - copy_to used the user provided size of the attribute
   and could copy data beyond the end of the kernel buffer into
   userspace.
 - copy_from didn't know the size of the kernel buffer and
   could have left kernel memory unexpectedly un-initialized.
 - copy_from did not use the user length to determine if the
   attribute data is inlined or not.

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