Move 32-bit compat support for FIODGNAME to the right place.

ioctl(2) commands only have meaning in the context of a file descriptor
so translating them in the syscall layer is incorrect.

The new handler users an accessor to retrieve/construct a pointer from
the last member of the passed structure and relies on type punning to
access the other member which requires no translation.

Reviewed by: kib
Approved by: re (rgrimes, gjb)
Obtained from: CheriBSD
Sponsored by: DARPA, AFRL
Differential Review: https://reviews.freebsd.org/D17388