LU-16714 utils: Clarify fd/fdv naming

LU-16714 utils: Clarify fd/fdv naming

The migrate code uses the deeply opaque 'fd' and 'fdv'
(file-descriptor-volatile) to refer to the source and
destination file descriptors.

Replace these with something that tells a casual reader
what's going on.

Test-parameters: trivial
Signed-off-by: Patrick Farrell <pfarrell@whamcloud.com>
Change-Id: I56f2dfe81bcd4eff1c168f093e8580b373dc1984
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/50546
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Timothy Day <timday@amazon.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>