autofs: make autofs_expire_direct() static

autofs_expire_direct() isn't used outside of fs/autofs/expire.c so make it
static.

Link: http://lkml.kernel.org/r/152937732944.21213.11821977712410930973.stgit@pluto.themaw.net


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