From: "Paul E. McKenney" <paulmck@us.ibm.com>

The vmtruncate() function shifts down by PAGE_CACHE_SHIFT, then calls
vmtruncate_list(), which deals in terms of PAGE_SHIFT instead.  Currently,
no harm done, since PAGE_CACHE_SHIFT and PAGE_SHIFT are identical.  Some
day they might not be, hence this patch.

I also took the liberty of modifying a hand-coded "if" that seems to
optimize for files that are not mapped to instead use unlikely().