mm/page_alloc: further fix __alloc_pages_bulk() return value

The author of commit b3b64ebd3822 ("mm/page_alloc: do bulk array
bounds check after checking populated elements") was possibly
confused by the mixture of return values throughout the function.

The API contract is clear that the function "Returns the number of pages
on the list or array." It does not list zero as a unique return value with
a special meaning.  Therefore zero is a plausible return value only if
@nr_pages is zero or less.

Clean up the return logic to make it clear that the returned value is
always the total number of pages in the array/list, not the number of
pages that were allocated during this call.

The only change in behavior with this patch is the value returned if
prepare_alloc_pages() fails.  To match the API contract, the number of
pages currently in the array/list is returned in this case.

The call site in __page_pool_alloc_pages_slow() also seems to be confused
on this matter.  It should be attended to by someone who is familiar with
that code.

[mel@techsingularity.net: Return nr_populated if 0 pages are requested]

Link: https://lkml.kernel.org/r/20210713152100.10381-4-mgorman@techsingularity.net
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Mel Gorman <mgorman@techsingularity.net>
Acked-by: Jesper Dangaard Brouer <brouer@redhat.com>
Cc: Desmond Cheong Zhi Xi <desmondcheongzx@gmail.com>
Cc: Zhang Qiang <Qiang.Zhang@windriver.com>
Cc: Yanfei Xu <yanfei.xu@windriver.com>
Cc: Matteo Croce <mcroce@microsoft.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

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