LoongArch: Set correct size for vDSO code mapping

The current size of vDSO code mapping is hardcoded to PAGE_SIZE. This
cannot work for 4KB page size after commit 18efd0b1 ("LoongArch:
vDSO: Wire up getrandom() vDSO implementation") because the code size
increases to 8KB. Thus set the code mapping size to its real size, i.e.
PAGE_ALIGN(vdso_end - vdso_start).

Fixes: 18efd0b1 ("LoongArch: vDSO: Wire up getrandom() vDSO implementation")
Reviewed-by: Xi Ruoyao <xry111@xry111.site>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>