userfaultfd: remove (VM_)BUG_ON()s

BUG_ON() is deprecated [1].  Convert all the BUG_ON()s and VM_BUG_ON()s to
use VM_WARN_ON_ONCE().

There are a few additional cases that are converted or modified:

- Convert the printk(KERN_WARNING ...) in handle_userfault() to use
  pr_warn().

- Convert the WARN_ON_ONCE()s in move_pages() to use VM_WARN_ON_ONCE(),
  as the relevant conditions are already checked in validate_range() in
  move_pages()'s caller.

- Convert the VM_WARN_ON()'s in move_pages() to VM_WARN_ON_ONCE(). These
  cases should never happen and are similar to those in mfill_atomic()
  and mfill_atomic_hugetlb(), which were previously BUG_ON()s.
  move_pages() was added later than those functions and makes use of
  VM_WARN_ON() as a replacement for the deprecated BUG_ON(), but.
  VM_WARN_ON_ONCE() is likely a better direct replacement.

- Convert the WARN_ON() for !VM_MAYWRITE in userfaultfd_unregister() and
  userfaultfd_register_range() to VM_WARN_ON_ONCE(). This condition is
  enforced in userfaultfd_register() so it should never happen, and can
  be converted to a debug check.

[1] https://www.kernel.org/doc/html/v6.15/process/coding-style.html#use-warn-rather-than-bug

Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Tal Zussman <[email protected]>
Cc: Al Viro <[email protected]>
Cc: Andrea Arcangeli <[email protected]>
Cc: Christian Brauner <[email protected]>
Cc: David Hildenbrand <[email protected]>
Cc: Jan Kara <[email protected]>
Cc: Jason A. Donenfeld <[email protected]>
Cc: Peter Xu <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

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