autofs4: indirect dentry must almost always be positive

We have been seeing mount requests comming to the automount daemon for
keys of the form "<map key>/<non key directory>" which are lookups for
invalid map keys.  But we can check for this in the kernel module and
return a fail immediately, without having to send a request to the daemon.

It is possible to recognise these requests are invalid based on whether
the request dentry is negative and its relation to the autofs file system
root.

For example, given the indirect multi-mount map entry:

idm1  \
    /mm1  <server>:/<path1>
    /mm2  <server>:/<path2>

For a request to mount idm1, IS_ROOT((idm1)->d_parent) will be always be
true and the dentry may be negative.  But directories idm1/mm1 and
idm1/mm2 will always be created as part of the mount request for idm1.  So
any mount request within idm1 itself must have a positive dentry otherwise
the map key is invalid.

In version 4 these multi-mount entries are all mounted and umounted as a
single request and in version 5 the directories idm1/mm1 and idm1/mm2 are
created and an autofs fs mounted on them to act as a mount trigger so the
above is also true.

This also holds true for the autofs version 4 pseudo direct mount feature.
 When this feature is used without the "--ghost" option automount(8) will
create internal submounts as we go down the map key paths which are
essentially normal indirect mounts for which the above holds.  If the
"--ghost" option is given the directories for map keys are created at
daemon startup so valid map entries correspond to postive dentries in the
autofs fs.

autofs version 5 direct mount maps are similar except that the IS_ROOT
check is not needed.  This has been addressed in a previous patch tittled
"autofs4 - detect invalid direct mount requests".

For example, given the direct multi-mount map entry:

/test/dm1  \
    /mm1  <server>:/<path1>
    /mm2  <server>:/<path2>

An autofs fs is mounted on /test/dm1 as a trigger mount and when a mount
is triggered for /test/dm1, the multi-mount offset directories
/test/dm1/mm1 and /test/dm1/mm2 are created and an autofs fs is mounted on
them to act as mount triggers.  So valid direct mount requests must always
have a positive dentry if they correspond to a valid map entry.

Signed-off-by: Ian Kent <raven@themaw.net>
Acked-by: Jeff Moyer <jmoyer@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>