xfs: introduce new file range commit ioctls

xfs: introduce new file range commit ioctls

This patch introduces two more new ioctls to manage atomic updates to
file contents -- XFS_IOC_START_COMMIT and XFS_IOC_COMMIT_RANGE.  The
commit mechanism here is exactly the same as what XFS_IOC_EXCHANGE_RANGE
does, but with the additional requirement that file2 cannot have changed
since some sampling point.  The start-commit ioctl performs the sampling
of file attributes.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>