autofs: update mount control expire desription with AUTOFS_EXP_FORCED

Describe AUTOFS_EXP_FORCED in addition to AUTOFS_EXP_IMMEDIATE in the
description of the AUTOFS_DEV_IOCTL_EXPIRE_CMD ioctl.

Link: http://lkml.kernel.org/r/155287084078.12593.15000931045413195778.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>

Documentation/filesystems/autofs-mount-control.txt

@@ -354,8 +354,10 @@ this ioctl is called until no further expire candidates are found.
 
 The call requires an initialized struct autofs_dev_ioctl with the
 ioctlfd field set to the descriptor obtained from the open call. In
-addition an immediate expire, independent of the mount timeout, can be
-requested by setting the how field of struct args_expire to 1. If no
+addition an immediate expire that's independent of the mount timeout,
+and a forced expire that's independent of whether the mount is busy,
+can be requested by setting the how field of struct args_expire to
+AUTOFS_EXP_IMMEDIATE or AUTOFS_EXP_FORCED, respectively . If no
 expire candidates can be found the ioctl returns -1 with errno set to
 EAGAIN.
 

Documentation/filesystems/autofs.txt

@@ -410,6 +410,11 @@ The available ioctl commands are:
      **AUTOFS_EXP_IMMEDIATE** causes `last_used` time to be ignored
      and objects are expired if the are not in use.
 
+     **AUTOFS_EXP_FORCED** causes the in use status to be ignored
+     and objects are expired ieven if they are in use. This assumes
+     that the daemon has requested this because it is capable of
+     performing the umount.
+
      **AUTOFS_EXP_LEAVES** will select a leaf rather than a top-level
      name to expire.  This is only safe when *maxproto* is 4.