mm/userfaultfd: support WP on multiple VMAs

mwriteprotect_range() errors out if [start, end) doesn't fall in one VMA. 
We are facing a use case where multiple VMAs are present in one range of
interest.  For example, the following pseudocode reproduces the error
which we are trying to fix:

- Allocate memory of size 16 pages with PROT_NONE with mmap
- Register userfaultfd
- Change protection of the first half (1 to 8 pages) of memory to
  PROT_READ | PROT_WRITE. This breaks the memory area in two VMAs.
- Now UFFDIO_WRITEPROTECT_MODE_WP on the whole memory of 16 pages errors
  out.

This is a simple use case where user may or may not know if the memory
area has been divided into multiple VMAs.

We need an implementation which doesn't disrupt the already present users.
So keeping things simple, stop going over all the VMAs if any one of the
VMA hasn't been registered in WP mode.  While at it, remove the un-needed
error check as well.

[[email protected]: s/VM_WARN_ON_ONCE/VM_WARN_ONCE/ to fix build]
Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Muhammad Usama Anjum <[email protected]>
Acked-by: Peter Xu <[email protected]>
Acked-by: David Hildenbrand <[email protected]>
Reported-by: Paul Gofman <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

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