Fix BUILD_BUG_ON()

Fix BUILD_BUG_ON()

As was noticed on the Linux side, using an array here isn't appropriate
if the condition is not a compile time constant - gcc allows such
arrays, and hence the intended effect of producing a compiler error is
not achieved in that case. Bit field widths do not know similar
language extensions, and hence always produce a compiler error.

Signed-off-by: Jan Beulich <jbeulich@novell.com>