Make sure no-one calls dentry_open with a NULL vfsmount argument and crap
out with a stacktrace otherwise.  A NULL file->f_vfsmnt has always been
problematic, but with the per-mount r/o tracking we can't accept anymore
at all.

[AV] the last place that passed NULL had been eliminated by the previous
patch (reiserfs xattr stuff)
Acked-by: Al Viro <viro@ZenIV.linux.org.uk>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Dave Hansen <haveblue@us.ibm.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>