autofs4 - fix missed case when changing to use struct path

In the recent change by Al Viro that changes verious subsystems
to use "struct path" one case was missed in the autofs4 module
which causes mounts to no longer expire.

Signed-off-by: Ian Kent <raven@themaw.net>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>