debugfs: rename start_creating() to debugfs_start_creating()

start_creating() is a generic name which I would like to use for a
function similar to simple_start_creating(), only not quite so simple.

debugfs is using this name which, though static, will cause complaints
if then name is given a different signature in a header file.

So rename it to debugfs_start_creating().

Signed-off-by: NeilBrown <neil@brown.name>
Signed-off-by: Christian Brauner <brauner@kernel.org>