userfaultfd: don't BUG_ON() if khugepaged yanks our page table

In the Linux kernel, the following vulnerability has been resolved:

userfaultfd: don't BUG_ON() if khugepaged yanks our page table

Since khugepaged was changed to allow retracting page tables in file

mappings without holding the mmap lock, these BUG_ON()s are wrong - get

rid of them.

We could also remove the preceding "if (unlikely(...))" block, but then we

could reach pte_offset_map_lock() with transhuge pages not just for file

mappings but also for anonymous mappings - which would probably be fine

but I think is not necessarily expected.