mm: walk_memory_range(): fix typo in comment

Fix a typo "end_pft" in the comment of walk_memory_range().

Signed-off-by: Toshi Kani <toshi.kani@hp.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>