btrfs: Fix a data space underflow warning

Even with quota disabled, generic/127 will trigger a kernel warning by
underflow data space info.

The bug is caused by buffered write, which in case of short copy, the
start parameter for btrfs_delalloc_release_space() is wrong, and
round_up/down() in btrfs_delalloc_release() extents the range to page
aligned, decreasing one more page than expected.

This patch will fix it by passing correct start.
Signed-off-by: Qu Wenruo <quwenruo@cn.fujitsu.com>
Signed-off-by: Chris Mason <clm@fb.com>

fs/btrfs/file.c

@@ -1604,13 +1604,18 @@ static noinline ssize_t __btrfs_buffered_write(struct file *file,
 				BTRFS_I(inode)->outstanding_extents++;
 				spin_unlock(&BTRFS_I(inode)->lock);
 			}
-			if (only_release_metadata)
+			if (only_release_metadata) {
 				btrfs_delalloc_release_metadata(inode,
 								release_bytes);
-			else
-				btrfs_delalloc_release_space(inode, pos,
+			} else {
+				u64 __pos;
+
+				__pos = round_down(pos, root->sectorsize) +
+					(dirty_pages << PAGE_CACHE_SHIFT);
+				btrfs_delalloc_release_space(inode, __pos,
 							     release_bytes);
 			}
+		}
 
 		release_bytes = dirty_pages << PAGE_CACHE_SHIFT;