stress2: exlock2.sh: Fix impact of previous '-U' removal

In commit "stress2: exlock2.sh: Preserve behavior after ps(1)'s '-U'
change" (1265d3b07eaa), I forgot that '-U' also has the side-effect of
implying '-x' by default. Now that '-U' has been removed, make '-x'
explicit.

Fixes: 1265d3b07eaa ("stress2: exlock2.sh: Preserve behavior after ps(1)'s '-U' change")
MFC after: 1 hour
Sponsored by: The FreeBSD Foundation

(cherry picked from commit f6e826891760a34ef69e1122da0ee378f7caf454)