fs/affs: add rename exchange

Process RENAME_EXCHANGE in affs_rename2() adding static
affs_xrename() based on affs_rename().

We remove headers from respective directories then
affect bh to other inode directory entries for swapping.

Signed-off-by: Fabian Frederick <fabf@skynet.be>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>