misc: privatize the FIEXCHANGE ioctl for now

misc: privatize the FIEXCHANGE ioctl for now

I'm abandoning (for now) efforts to bring atomic file content exchanges
to the VFS.  The goal here is to reduce friction in getting online fsck
merged, so Dave and I want to take this back to being a private XFS
ioctl so we can explore with it for a while before committing it to the
stable KABI.

Shift all the existing FIEXCHANGE usage to XFS_IOC_EXCHANGE_RANGE, and
try to pick it up from xfs_fs_staging.h if the system xfslibs-dev
package has such an animal.

Signed-off-by: "Darrick J. Wong" <djwong@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Zorro Lang <zlang@kernel.org>