linux: Update the i386/linux vdso deinitialization routine
This was missed in commit 0fc8a796722 ("linux: Unmap the VDSO page when
unloading").
Reported by: Mark Millard
MFC with: 0fc8a796722
(cherry picked from commit aa5fef60bf663d31cddf53d7a8c6856d5e4672c0)