arm: mm: use __pfn_to_section() to get mem_section

Replace the open-coded '__nr_to_section(pfn_to_section_nr(pfn))' in
pfn_valid() with a more concise call to '__pfn_to_section(pfn)'.

No functional change.

Signed-off-by: Guixiong Wei <guixiongwei@gmail.com>
Link: https://lore.kernel.org/r/20200430161858.11379-1-guixiongwei@gmail.com


Signed-off-by: Will Deacon <will@kernel.org>