Changes to copy_file_range for 5.3 from Dave and Amir:

- Create a generic copy_file_range handler and make individual
  filesystems responsible for calling it (i.e. no more assuming that
  do_splice_direct will work or is appropriate)
- Refactor copy_file_range and remap_range parameter checking where they
  are the same
- Install missing copy_file_range parameter checking(!)
- Remove suid/sgid and update mtime like any other file write
- Change the behavior so that a copy range crossing the source file's
  eof will result in a short copy to the source file's eof instead of
  EINVAL
- Permit filesystems to decide if they want to handle cross-superblock
  copy_file_range in their local handlers.
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