mm/rmap: replace BUG_ON(anon_vma->degree) with VM_WARN_ON

This check effectively catches anon vma hierarchy inconsistence and some
vma corruptions.  It was effective for catching corner cases in anon vma
reusing logic.  For now this code seems stable so check could be hidden
under CONFIG_DEBUG_VM and replaced with WARN because it's not so fatal.

Signed-off-by: Konstantin Khlebnikov <khlebnikov@yandex-team.ru>
Suggested-by: Vasily Averin <vvs@virtuozzo.com>
Acked-by: Vlastimil Babka <vbabka@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>