LU-17463 osc: add option to disable page cache shrinker

LU-17463 osc: add option to disable page cache shrinker

The pages mapped into VM_LOCKED [mlocked()ed] VMAs are unevictable
pages. Those pages are marked with PG_mlocked.
However, page cache shrinker in Lustre treats all cached pages
equally even some of them are unevictable. It may evict mlocked
pages by mlock() or mlockall() calls wrongly.

This patch adds an tunable option to enable or disable page cache
shrinker:
- osc.*.enable_page_cache_shrink
It is enabled by default.

Lustre-Change: https://review.whamcloud.com/53795
Lustre-Commit: d90ce0aab10ee8856140720cd71935da6877a5ab

Signed-off-by: Qian Yingjin <qian@ddn.com>
Change-Id: I23ebf6d438a71c7917b0cb3375407a64587e15db
Reviewed-on: https://review.whamcloud.com/c/ex/lustre-release/+/54754
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>