pagecache: support percpu refcount to imporve performance

euleros inclusion
category: feature
feature: pagecache percpu refcount
bugzilla: 31398
CVE: NA

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The pagecache manages the file physical pages, and the life cycle of
page is managed by atomic counting. With the increasing number of cpu
cores, the cost of atomic counting is very large when reading file
pagecaches at large concurrent.

For example, when running nginx http application, the biggest hotspot is
found in the atomic operation of find_get_entry():

 11.94% [kernel] [k] find_get_entry
  7.45% [kernel] [k] do_tcp_sendpages
  6.12% [kernel] [k] generic_file_buffered_read

So we using the percpu refcount mechanism to fix this problem. and the
test result show that the read performance of nginx http can be improved
by 100%：

  worker   original(requests/sec)   percpu(requests/sec)   imporve
  64       759656.87                1627088.95             114.2%

Notes: we use page->lru to save percpu...