mm: use __pfn_to_section() instead of open coding it

[ Upstream commit f1dc0db2 ]

It is defined in the same file just a few lines above.

Link: https://lkml.kernel.org/r/4598487.Rc0NezkW7i@mobilepool36.emlix.com


Signed-off-by: Rolf Eike Beer <eb@emlix.com>
Reviewed-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Stable-dep-of: 5ec8e8ea ("mm/sparsemem: fix race in accessing memory_section->usage")
Signed-off-by: Sasha Levin <sashal@kernel.org>