xfs: fix regression in "cleanup xfs_dir2_block_getdents"

Commit 263dde86 ("xfs: cleanup xfs_dir2_block_getdents") introduced
a getdents regression, when it converted the pointer arithmetics to
offset calculations: offset is updated in the loop already for the next
iteration, but the updated offset value is used incorrectly in two
places, where we should have used the not-yet-updated value.

This caused for example "git clean -ffdx" failures to cleanup certain
directory structures when running in a container.

Fix the regression by making sure we use proper offset in the loop body.
Thanks to Christoph Hellwig for suggestion how to best fix the code.

Cc: Christoph Hellwig <hch@lst.de>
Fixes: 263dde86 ("xfs: cleanup xfs_dir2_block_getdents")
Signed-off-by: Tommi Rantala <tommi.t.rantala@nokia.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>