vfs: add cross-rename

If flags contain RENAME_EXCHANGE then exchange source and destination files.
There's no restriction on the type of the files; e.g. a directory can be
exchanged with a symlink.

Signed-off-by: Miklos Szeredi <mszeredi@suse.cz>
Reviewed-by: Jan Kara <jack@suse.cz>
Reviewed-by: J. Bruce Fields <bfields@redhat.com>

include/uapi/linux/fs.h

@@ -36,6 +36,7 @@
 #define SEEK_MAX	SEEK_HOLE
 
 #define RENAME_NOREPLACE	(1 << 0)	/* Don't overwrite target */
+#define RENAME_EXCHANGE		(1 << 1)	/* Exchange source and dest */
 
 struct fstrim_range {
 	__u64 start;