mm/mmap.c: rb_parent is not necessary in __vma_link_list()

mm/mmap.c: rb_parent is not necessary in __vma_link_list()

Now we use rb_parent to get next, while this is not necessary.

When prev is NULL, this means vma should be the first element in the list.
Then next should be current first one (mm->mmap), no matter whether we
have parent or not.

After removing it, the code shows the beauty of symmetry.

Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Wei Yang <[email protected]>
Acked-by: Andrew Morton <[email protected]>
Cc: Mel Gorman <[email protected]>
Cc: Vlastimil Babka <[email protected]>
Cc: Matthew Wilcox <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>