hugetlb: Replace BUG() with BUILD_BUG() for dummy definitions.

The file linux/hugetlb.h has many places where dummy symbols were
defined so that the main source code would contain fewer:

    #ifdef CONFIG_HUGETLBFS

or

    #ifdef CONFIG_TRANSPARENT_HUGEPAGE

If there were any misuse of these symbols, the only symptom would be
an OOPS at runtime.  Change the BUG() to BUILD_BUG() to catch
any such abuse at compile time instead.

Signed-off-by: David Daney <david.daney@cavium.com>
backported to 2.6.35 from linux 3.2-rc3+

Signed-off-by: nobodyAtall <tsogkas@ceid.upatras.gr>
Signed-off-by: garwedgess <garethwilliams21@gmail.com>