lib/utmp.c: get_current_utmp(): Use simple assignment instead of memcpy(3)

lib/utmp.c: get_current_utmp(): Use simple assignment instead of memcpy(3)

memcpy(3) is overkill, and much more dangerous than simple assignment.
Simple assignment adds type safety, and removes any possibility of
buffer overflow due to accidentally specifying a wrong size.

Signed-off-by: Alejandro Colomar <alx@kernel.org>