mm/highmem.c: fix coding style issue

Delete/add some blank lines and some blank spaces

Link: https://lkml.kernel.org/r/20210311095015.14277-1-songqiang@uniontech.com


Signed-off-by: songqiang <songqiang@uniontech.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>