FreeBSD: Add posix_fadvise(POSIX_FADV_WILLNEED) support
As commit 320f0c6 did for Linux, connect POSIX_FADV_WILLNEED
up to dmu_prefetch() on FreeBSD.
While there, fix portability problems in tests/functional/fadvise.
- Instead of relying on the numerical values of POSIX_FADV_XXX macros, accept macro names as arguments to the file_fadvise program. (The numbers happen to match on Linux and FreeBSD, but future systems may vary and it seems a little strange/raw to count on that.)
- For implementation reasons, SEQUENTIAL doesn't reach ZFS via FreeBSD VFS currently (perhaps something that should be investigated in FreeBSD). Since on Linux we're treating SEQUENTIAL and WILLNEED the same, it doesn't really matter which one we use, so switch the test over to WILLNEED exercise the new prefetch code on both OSes the same way.
Reviewed-by: Mateusz Guzik <mjg@FreeBSD.org>
Reviewed-by: Fedor Uporov <fuporov.vstack@gmail.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Signed-off-by: Thomas Munro <tmunro@FreeBSD.org>
Co-authored-by: Alexander Motin <mav@FreeBSD.org>
Closes #17379