fasync: Fix placement of FASYNC flag comment

In commit f7347ce4 ("fasync: re-organize fasync entry insertion to
allow it under a spinlock") Arnd took an earlier patch of mine that had
the comment about the FASYNC flag above the wrong function.

When the fasync_add_entry() function was split to introduce the new
fasync_insert_entry() helper function, the code that actually cares
about the FASYNC bit moved to that new helper.

So just move the comment to the right point.

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>