mmap2: Clarify MAP_POPULATE

mmap2: Clarify MAP_POPULATE

David Rientjes has noticed that MAP_POPULATE wording might promise
much more than the kernel actually provides and intend to provide.
The primary usage of the flag is to pre-fault the range. There is
no guarantee that no major faults will happen later on. The pages
might have been reclaimed by the time the process tries to access
them.

Reviewed-by: Eric B Munson <emunson@akamai.com>
Signed-off-by: Michal Hocko <mhocko@suse.cz>
Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>