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

Vulnerability Description

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.

References

• https://git.kernel.org/stable/c/4a594acc12d5954cdc71d4450a386748bf3d136a
• https://git.kernel.org/stable/c/db978287e908d48b209e374b00d847b2d785e0a9
• https://git.kernel.org/stable/c/4828d207dc5161dc7ddf9a4f6dcfd80c7dd7d20a