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This patch adds POSIX_FADV_NOREUSE to vma_has_recency() so that the LRU
algorithm can ignore access to mapped files marked by this flag.

The advantages of POSIX_FADV_NOREUSE are:
1. Unlike MADV_SEQUENTIAL and MADV_RANDOM, it does not alter the
   default readahead behavior.
2. Unlike MADV_SEQUENTIAL and MADV_RANDOM, it does not split VMAs and
   therefore does not take mmap_lock.
3. Unlike MADV_COLD, setting it has a negligible cost, regardless of
   how many pages it affects.

Its limitations are:
1. Like POSIX_FADV_RANDOM and POSIX_FADV_SEQUENTIAL, it currently does
   not support range. IOW, its scope is the entire file.
2. It currently does not ignore access through file descriptors.
   Specifically, for the active/inactive LRU, given a file page shared
   by two users and one of them having set POSIX_FADV_NOREUSE on the
   file, this page will be activated upon the second user accessing
   it. This corner case can be covered by checking POSIX_FADV_NOREUSE
   before cal...