vfs: add helpers to get root and pwd

Add three helpers that retrieve a refcounted copy of the root and cwd
from the supplied fs_struct.

 get_fs_root()
 get_fs_pwd()
 get_fs_root_and_pwd()

Signed-off-by: Miklos Szeredi <mszeredi@suse.cz>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
diff --git a/fs/namei.c b/fs/namei.c
index 13ff4ab..17ea76bf 100644
--- a/fs/namei.c
+++ b/fs/namei.c
@@ -483,13 +483,8 @@
 
 static __always_inline void set_root(struct nameidata *nd)
 {
-	if (!nd->root.mnt) {
-		struct fs_struct *fs = current->fs;
-		read_lock(&fs->lock);
-		nd->root = fs->root;
-		path_get(&nd->root);
-		read_unlock(&fs->lock);
-	}
+	if (!nd->root.mnt)
+		get_fs_root(current->fs, &nd->root);
 }
 
 static int link_path_walk(const char *, struct nameidata *);
@@ -1015,11 +1010,7 @@
 		nd->path = nd->root;
 		path_get(&nd->root);
 	} else if (dfd == AT_FDCWD) {
-		struct fs_struct *fs = current->fs;
-		read_lock(&fs->lock);
-		nd->path = fs->pwd;
-		path_get(&fs->pwd);
-		read_unlock(&fs->lock);
+		get_fs_pwd(current->fs, &nd->path);
 	} else {
 		struct dentry *dentry;