mm: pagewalk: assert write mmap lock only for walking the user page tables

The 8782fb61cc848 ("mm: pagewalk: Fix race between unmap and page walker")
introduces an assertion to walk_page_range_novma() to make all the users
of page table walker is safe.  However, the race only exists for walking
the user page tables.  And it is ridiculous to hold a particular user mmap
write lock against the changes of the kernel page tables.  So only assert
at least mmap read lock when walking the kernel page tables.  And some
users matching this case could downgrade to a mmap read lock to relief the
contention of mmap lock of init_mm, it will be nicer in hugetlb (only
holding mmap read lock) in the next patch.

Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Muchun Song <[email protected]>
Acked-by: Mike Kravetz <[email protected]>
Cc: Kefeng Wang <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

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