mm/gup: decrement head page once for group of subpages

Rather than decrementing the head page refcount one by one, we walk the
page array and checking which belong to the same compound_head.  Later on
we decrement the calculated amount of references in a single write to the
head page.  To that end switch to for_each_compound_head() does most of
the work.

set_page_dirty() needs no adjustment as it's a nop for non-dirty head
pages and it doesn't operate on tail pages.

This considerably improves unpinning of pages with THP and hugetlbfs:

 - THP

   gup_test -t -m 16384 -r 10 [-L|-a] -S -n 512 -w
   PIN_LONGTERM_BENCHMARK (put values): ~87.6k us -> ~23.2k us

- 16G with 1G huge page size

  gup_test -f /mnt/huge/file -m 16384 -r 10 [-L|-a] -S -n 512 -w
  PIN_LONGTERM_BENCHMARK: (put values): ~87.6k us -> ~27.5k us

Link: https://lkml.kernel.org/r/20210212130843.13865-3-joao.m.martins@oracle.com
Signed-off-by: Joao Martins <joao.m.martins@oracle.com>
Reviewed-by: John Hubbard <jhubbard@nvidia.com>
Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>
Cc: Christoph Hellwig <hch@infradead.org>
Cc: Doug Ledford <dledford@redhat.com>
Cc: Matthew Wilcox <willy@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

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