mm: decline to manipulate the refcount on a slab page

Slab pages now have a refcount of 0, so nobody should be trying to
manipulate the refcount on them.  Doing so has little effect; the object
could be freed and reallocated to a different purpose, although the slab
itself would not be until the refcount was put making it behave rather
like TYPESAFE_BY_RCU.

Unfortunately, __iov_iter_get_pages_alloc() does take a refcount.  Fix
that to not change the refcount, and make put_page() silently not change
the refcount.  get_page() warns so that we can fix any other callers that
need to be changed.

Long-term, networking needs to stop taking a refcount on the pages that it
uses and rely on the caller to hold whatever references are necessary to
make the memory stable.  In the medium term, more page types are going to
hav a zero refcount, so we'll want to move get_page() and put_page() out
of line.

Link: https://lkml.kernel.org/r/[email protected]
Fixes: 9aec2fb0fd5e (slab: allocate frozen pages)
Signed-off-by: Matthew Wilcox (Oracle) <[email protected]>
Reported-by: Hannes Reinecke <[email protected]>
Closes: https://lore.kernel.org/all/[email protected]/
Acked-by: Vlastimil Babka <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

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