Cache root in nameidata

New field: nd->root.  When pathname resolution wants to know the root,
check if nd->root.mnt is non-NULL; use nd->root if it is, otherwise
copy current->fs->root there.  After path_walk() is finished, we check
if we'd got a cached value in nd->root and drop it.  Before calling
path_walk() we should either set nd->root.mnt to NULL *or* copy (and
pin down) some path to nd->root.  In the latter case we won't be
looking at current->fs->root at all.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

include/linux/namei.h

@@ -18,6 +18,7 @@ enum { MAX_NESTED_LINKS = 8 };
 struct nameidata {
 	struct path	path;
 	struct qstr	last;
+	struct path	root;
 	unsigned int	flags;
 	int		last_type;
 	unsigned	depth;