arm64: mm: simplify the page end calculation in __create_pgd_mapping()

Calculate the page-aligned end address more simply.

The local variable, "length" is unneeded.

Reviewed-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>