Smack: fuse mount hang fix

The d_instantiate hook for Smack can hang on the root inode of a
filesystem if the file system code has not really done all the set-up.
Fuse is known to encounter this problem.

This change detects an attempt to instantiate a root inode and addresses
it early in the processing, before any attempt is made to do something
that might hang.

Signed-off-by: Casey Schaufler <[e-mail]>
Tested-by: Luiz Fernando N. Capitulino <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

security/smack/smack_lsm.c

···
 	final = sbsp->smk_default;
 
 	/*
+	 * If this is the root inode the superblock
+	 * may be in the process of initialization.
+	 * If that is the case use the root value out
+	 * of the superblock.
+	 */
+	if (opt_dentry->d_parent == opt_dentry) {
+		isp->smk_inode = sbsp->smk_root;
+		isp->smk_flags |= SMK_INODE_INSTANT;
+		goto unlockandout;
+	}
+
+	/*
 	 * This is pretty hackish.
 	 * Casey says that we shouldn't have to do
 	 * file system specific code, but it does help