i40e: use WARN_ONCE to replace the commented BUG_ON size check

We don't really want to use BUG_ON here since that would completely
crash the kernel, thus the reason we commented it out. We *can't* use
BUILD_BUG_ON because at least now (a) the sizes aren't constant (we are
fixing this) and (b) not all compilers are smart enough to understand
that "p - data" is a constant.

Instead, just use a WARN_ONCE so that the first time we end up with an
incorrect size we will dump a stack trace and a message, hopefully
highlighting the issues early in testing.

Signed-off-by: Jacob Keller <jacob.e.keller@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>