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 <casey@schaufler-ca.com>
Tested-by: Luiz Fernando N. Capitulino <lcapitulino@mandriva.com.br>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>