linux: implement filesystem-side copy/clone functions for EL7

Redhat have backported copy_file_range and clone_file_range to the EL7
kernel using an "extended file operations" wrapper structure. This
connects all that up to let cloning work there too.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Kay Pedersen <mail@mkwg.de>
Signed-off-by: Rob Norris <rob.norris@klarasystems.com>
Sponsored-By: OpenDrives Inc.
Sponsored-By: Klara Inc.
Closes #15050