Btrfs: Fix file clone when source offset is not 0

Suppose:
- the source extent is: [0, 100]
- the src offset is 10
- the clone length is 90
- the dest offset is 0

This statement:

	new_key.offset = key.offset + destoff - off

will produce such an extent for the dest file:

	[ino, BTRFS_EXTENT_DATA_KEY, -10]

, which is obviously wrong.

Signed-off-by: Li Zefan <lizf@cn.fujitsu.com>