linux: implement filesystem-side copy/clone functions

This implements the Linux VFS ops required to service the file
copy/clone APIs:

.copy_file_range    (4.5+)
.clone_file_range   (4.5-4.19)
.dedupe_file_range  (4.5-4.19)
.remap_file_range   (4.20+)

Note that dedupe_file_range() and remap_file_range(REMAP_FILE_DEDUP) are
hooked up here, but are not implemented yet.

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