mm/highmem.c: clean up endif comments

mm/highmem.c: clean up endif comments

The #endif at the end of the file matches up with the '#if
defined(HASHED_PAGE_VIRTUAL)' on line 374.  Not the CONFIG_HIGHMEM #if
earlier.

Fix comments on both of the #endif's to indicate the correct end of
blocks for each.

Signed-off-by: Ira Weiny <ira.weiny@intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Reviewed-by: Andrew Morton <akpm@linux-foundation.org>
Reviewed-by: Mike Rapoport <rppt@linux.ibm.com>
Link: https://lkml.kernel.org/r/20200819184635.112579-1-ira.weiny@intel.com
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>