mm: Support fadvise without CONFIG_MMU

Commit d3ac21ca

 ("mm: Support compiling
out madvise and fadvise") incorrectly made fadvise conditional on
CONFIG_MMU.  (The merged branch unintentionally incorporated v1 of the
patch rather than the fixed v2.)  Apply the delta from v1 to v2, to
allow fadvise without CONFIG_MMU.

Reported-by: Johannes Weiner <hannes@cmpxchg.org>
Signed-off-by: Josh Triplett <josh@joshtriplett.org>