Fix for shmem_truncate_range() BUG_ON()

Ran into BUG() while doing madvise(REMOVE) testing.  If we are punching a
hole into shared memory segment using madvise(REMOVE) and the entire hole
is below the indirect blocks, we hit following assert.

            BUG_ON(limit <= SHMEM_NR_DIRECT);
Signed-off-by: Badari Pulavarty <pbadari@us.ibm.com>
Forwarded-by: Jordan Neumeyer
Signed-off-by: Adrian Bunk <bunk@stusta.de>

mm/shmem.c

@@ -510,7 +510,12 @@ static void shmem_truncate_range(struct inode *inode, loff_t start, loff_t end)
 			size = SHMEM_NR_DIRECT;
 		nr_swaps_freed = shmem_free_swp(ptr+idx, ptr+size);
 	}
-	if (!topdir)
+
+	/*
+	 * If there are no indirect blocks or we are punching a hole
+	 * below indirect blocks, nothing to be done.
+	 */
+	if (!topdir || (punch_hole && (limit <= SHMEM_NR_DIRECT)))
 		goto done2;
 
 	BUG_ON(limit <= SHMEM_NR_DIRECT);