Use mlock2() instead of mlock() on Linux and use mlockall(MCL_FUTURE).

[ghuls]

Use mlock2() instead of mlock() on Linux:

• mlock2() allows a third argument MLOCK_ONFAULT, which allow to lock the pages in a range before touching them, so mlock2() can be called before touching all pages instead of after.
• "vmtouch -t -l" some_file would not keep the pages in cache after touching them, so at the lock() step they needed to be read again from disk. After this patch files are cached twice as fast on this system.

Use mlockall(MCL_FUTURE) instead of mlockall(MCL_CURRENT):

• Set mlockall(MCL_FUTURE) before touching pages to have similar behaviour as the mlock2() approach above.