Introduce a new ioctl to handle exchanging ranges of bytes
between files.  The goal here is to perform the exchange atomically with
respect to applications -- either they see the file contents before the
exchange or they see that A-B is now B-A, even if the kernel crashes.

My original goal with all this code was to make it so that online repair
can build a replacement directory or xattr structure in a temporary file
and commit the repair by atomically exchanging all the data blocks
between the two files.  However, I needed a way to test this mechanism
thoroughly, so I've been evolving an ioctl interface since then.
Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>