Btrfs: fix off-by-one error of the same page check in btrfs_punch_hole()

(start + len) is the start of the adjacent extent, not the end of the current
extent, so we should not use it to check the hole is on the same page or not.
Signed-off-by: NMiao Xie <miaox@cn.fujitsu.com>
Signed-off-by: NChris Mason <chris.mason@fusionio.com>

fs/btrfs/file.c

@@ -1867,8 +1867,8 @@ static int btrfs_punch_hole(struct inode *inode, loff_t offset, loff_t len)
 	u64 drop_end;
 	int ret = 0;
 	int err = 0;
-	bool same_page = (offset >> PAGE_CACHE_SHIFT) ==
-		((offset + len) >> PAGE_CACHE_SHIFT);
+	bool same_page = ((offset >> PAGE_CACHE_SHIFT) ==
+			  ((offset + len - 1) >> PAGE_CACHE_SHIFT));
 
 	btrfs_wait_ordered_range(inode, offset, len);