kernfs: fix reference to renamed function

commit c637b8ac ("kernfs:
 s/sysfs/kernfs/ in internal functions and whatever is left")
renamed kernfs_file_open to kernfs_fop_open, but didn't update the
comment referencing it.

Signed-off-by: Ahelenia Ziemiańska <nabijaczleweli@nabijaczleweli.xyz>
Acked-by: Tejun Heo <tj@kernel.org>
Link: https://lore.kernel.org/r/4f2wybrepigxjpuxj4bdkh3qmksetfioedit2bdrswf6b75ebb@tarta.nabijaczleweli.xyz


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>