linux(4): Handle special case for regular futex in handle_futex_death().

Handle some races in handle_futex_death() which can prevents a wakeup of
potential waiters which can cause these waiters to block forever.

Differential Revision: https://reviews.freebsd.org/D31280
MFC after: 2 weeks

(cherry picked from commit b59cf25eac06c1feb2d35a5a3d8a089ae62fd0df)