hugetlbfs: fix i_blocks accounting

As reported in Red Hat bz #509671, i_blocks for files on hugetlbfs get
accounting wrong when doing something like:

   $ > foo
   $ date  > foo
   date: write error: Invalid argument
   $ /usr/bin/stat foo
     File: `foo'
     Size: 0          Blocks: 18446744073709547520 IO Block: 2097152 regular
...

This is because hugetlb_unreserve_pages() is unconditionally removing
blocks_per_huge_page(h) on each call rather than using the freed amount.
If there were 0 blocks, it goes negative, resulting in the above.

This is a regression from commit a5516438959d90b071ff0a484ce4f3f523dc3152
("hugetlb: modular state for hugetlb page size")

which did:

-	inode->i_blocks -= BLOCKS_PER_HUGEPAGE * freed;
+	inode->i_blocks -= blocks_per_huge_page(h);

so just put back the freed multiplier, and it's all happy again.

Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Acked-by: Andi Kleen <andi@firstfloor.org>
Cc: William Lee Irwin III <wli@holomorphy.com>
Cc: <stable@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/hugetlb.c

@@ -2370,7 +2370,7 @@ void hugetlb_unreserve_pages(struct inode *inode, long offset, long freed)
 	long chg = region_truncate(&inode->i_mapping->private_list, offset);
 
 	spin_lock(&inode->i_lock);
-	inode->i_blocks -= blocks_per_huge_page(h);
+	inode->i_blocks -= (blocks_per_huge_page(h) * freed);
 	spin_unlock(&inode->i_lock);
 
 	hugetlb_put_quota(inode->i_mapping, (chg - freed));