mm/mmap.c: __vma_adjust(): suppress uninitialized var warning

The code is OK, but it fools gcc.

mm/mmap.c:802 __vma_adjust() error: uninitialized symbol 'next_next'.

Fixes: 524e00b3 ("mm: remove rb tree.")
Reported-by: kernel test robot <lkp@intel.com>
Cc: Liam R. Howlett <Liam.Howlett@Oracle.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>