ocfs2: Remove overzealous BUG_ON()

The truncate code was never supposed to BUG() on an allocator it doesn't
know about, but rather to ignore it. Right now, this does nothing, but when
we change our allocation paths to use all suballocator files, this will
allow current versions of the fs module to work fine.

Signed-off-by: Mark Fasheh <mark.fasheh@oracle.com>