autofs: add AUTOFS_EXP_FORCED flag

The userspace automount(8) daemon is meant to perform a forced expire when
sent a SIGUSR2.

But since the expiration is routed through the kernel and the kernel
doesn't send an expire request if the mount is busy this hasn't worked at
least since autofs version 5.

Add an AUTOFS_EXP_FORCED flag to allow implemention of the feature and
bump the protocol version so user space can check if it's implemented if
needed.

Link: http://lkml.kernel.org/r/152937734715.21213.6594007182776598970.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>