LU-14003 pcc: convert mapping pagecache for mmap

LU-14003 pcc: convert mapping pagecache for mmap

In the PCC mmap implementation, it will replace the mapping of
the PCC copy with the one of the Lustre file when do mmap() to
make the mmapped region (vma) link into the mapping of the
Lustre file not the mapping of the PCC copy.
At this time, in the old design the pagecache in the original
mapping of the PCC copy is simply dropped as the mapping of each
page is different after the replacement of the mapping.

This may have negative impact on the mmap performance.
The reason is that during PCC attach it will write the data from
Lustre into PCC copy in buffered I/O mode, these data will keep
in pagecache and managed by the mapping of the PCC copy if there
is enough system memory. Then for the latter mmap, the page fault
could directly read data from the pagecache to speed up the mmap
operation.
If drop these pagecahe due to the different mapping of each pages,
the page fault must read page from the disk and may result in bad
performance.

To make full use of these pagecache of the PCC copy, during mmap
call, it can first remove the page from the original mapping of
the PCC copy, and then convert and add it into the mapping of the
Lustre file. By this way, all pagecaches are converted and can be
reused for the latter page fault.

Test-Parameters: clientcount=3 testlist=sanity-pcc,sanity-pcc,sanity-pcc
Signed-off-by: Qian Yingjin <qian@ddn.com>
Change-Id: I1591937543d7d31b8811ec62088accd0070d7d37
Reviewed-on: https://review.whamcloud.com/41924
Reviewed-by: Wang Shilong <wshilong@whamcloud.com>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>