vfs: for usbfs, etc. internal vfsmounts ->mnt_sb->s_root == ->mnt_root

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

arch/ia64/kernel/perfmon.c

@@ -2228,7 +2228,7 @@ pfm_alloc_file(pfm_context_t *ctx)
 	/*
 	 * allocate a new dcache entry
 	 */
-	path.dentry = d_alloc(pfmfs_mnt->mnt_sb->s_root, &this);
+	path.dentry = d_alloc(pfmfs_mnt->mnt_root, &this);
 	if (!path.dentry) {
 		iput(inode);
 		return ERR_PTR(-ENOMEM);