automount_mount calls run_command which may trigger an automount
again. This results in an endless loop. A simple way to trigger
this is:

mkdir /x; automount /x false; cd /x; something

Use a static variable to detect if we are currently in automount_mount()
and bail out if we are.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>