linux(4): Futex address must be 32-bit aligned.

Linux futex documentation explicitly states that EINVAL is returned if
the futex is not 4-byte aligned. Check futex alignment as a Linux do
and return EINVAL.

Differential Revision: https://reviews.freebsd.org/D31279
MFC after: 2 weeks

(cherry picked from commit dad107705667dfca89db796c1c075a782f4666d4)