IB/ipath: Fix potential buffer overrun in sending diag packet routine

Guard against a potential buffer overrun.  The size to read from the
user is passed in, and due to the padding that needs to be taken into
account, as well as the place holder for the ICRC it is possible to
overflow the 32bit value which would cause more data to be copied from
user space than is allocated in the buffer.

Cc: <stable@vger.kernel.org>
Reported-by: Nico Golde <nico@ngolde.de>
Reported-by: Fabian Yamaguchi <fabs@goesec.de>
Reviewed-by: Mike Marciniszyn <mike.marciniszyn@intel.com>
Signed-off-by: Dennis Dalessandro <dennis.dalessandro@intel.com>
Signed-off-by: Roland Dreier <roland@purestorage.com>