Refactor all the reflink preparation steps into a separate helper
that we'll use to land all the upcoming fixes for insufficient input
checks.

This rework also moves the invalidation of the destination range to
the prep function so that it is done before the range is remapped.
This ensures that nobody can access the data in range being remapped
until the remap is complete.

[dgc: fix xfs_reflink_remap_prep() return value and caller check to
handle vfs_clone_file_prep_inodes() returning 0 to mean "nothing to
do". ]

[dgc: make sure length changed by vfs_clone_file_prep_inodes() gets
propagated back to XFS code that does the remapping. ]
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>