UPSTREAM: mm/Kconfig: fix URL for zsmalloc benchmark

(cherry-pick from commit 2216ee853017f9c9371106c5c02d4fe42f61cbfa)

The help text for CONFIG_PGTABLE_MAPPING has an incorrect URL.  While
we're at it, remove the unnecessary footnote notation.

Bug: 25951511

Change-Id: Ia2eb06b2a5d29960b51f0b6558ef5041fd9c03fa
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
Acked-by: Minchan Kim <minchan@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>