mm/filemap.c: remove bogus VM_BUG_ON

It is not safe to check page->index without holding the page lock.  It
can be changed if the page is moved between the swap cache and the page
cache for a shmem file, for example.  There is a VM_BUG_ON below which
checks page->index is correct after taking the page lock.

Link: https://lkml.kernel.org/r/20210818144932.940640-1-willy@infradead.org
Fixes: 5c211ba2

 ("mm: add and use find_lock_entries")
Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Reported-by:  <syzbot+c87be4f669d920c76330@syzkaller.appspotmail.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>