linux_ioctl: Refactor some v4l2 struct converters

According to the C standard, it is invalid to copy beyond the end of an
object, even if that object is obviously a member of a larger object (a
struct, in this case).

Appease the standard and Coverity by refactoring the copy in a
straightforward way. No functional change.

Reported by: Coverity (CWE-120)
CIDs: 1007819, 1007820, 1007821, 1007822, 1009668, 1009669
Security: no (false positive detection)
Sponsored by: Dell EMC Isilon