linux(4): Replace copyin() by fueword32() in handle_futex_death().

According to fetch(9) fueword facility designed to fetch atomically
small amount of data from user space.

Reviewed by: kib
Differential Revision: https://reviews.freebsd.org/D31239
MFC after: 2 weeks

(cherry picked from commit cb01cc4a1020d7916bcf0f1e774156745c387409)