NFS: don't try to cross a mountpount when there isn't one there.

NFS: don't try to cross a mountpount when there isn't one there.

consider the sequence of commands:
 mkdir -p /import/nfs /import/bind /import/etc
 mount --bind / /import/bind
 mount --make-private /import/bind
 mount --bind /import/etc /import/bind/etc

 exportfs -o rw,no_root_squash,crossmnt,async,no_subtree_check localhost:/
 mount -o vers=4 localhost:/ /import/nfs
 ls -l /import/nfs/etc

You would not expect this to report a stale file handle.
Yet it does.

The manipulations under /import/bind cause the dentry for
/etc to get the DCACHE_MOUNTED flag set, even though nothing
is mounted on /etc.  This causes nfsd to call
nfsd_cross_mnt() even though there is no mountpoint.  So an
upcall to mountd for "/etc" is performed.

The 'crossmnt' flag on the export of / causes mountd to
report that /etc is exported as it is a descendant of /.  It
assumes the kernel wouldn't ask about something that wasn't
a mountpoint.  The filehandle returned identifies the
filesystem and the inode number of /etc.

When this filehandle is presented to rpc.mountd, via
"nfsd.fh", the inode cannot be found associated with any
name in /etc/exports, or with any mountpoint listed by
getmntent().  So rpc.mountd says the filehandle doesn't
exist. Hence ESTALE.

This is fixed by teaching nfsd not to trust DCACHE_MOUNTED
too much.  It is just a hint, not a guarantee.
Change nfsd_mountpoint() to return '1' for a certain mountpoint,
'2' for a possible mountpoint, and 0 otherwise.

Then change nfsd_crossmnt() to check if follow_down()
actually found a mountpount and, if not, to avoid performing
a lookup if the location is not known to certainly require
an export-point.

Signed-off-by: NeilBrown <neilb@suse.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>