vfs: Fix vfsmount_lock imbalance in path_init()

vfs: Fix vfsmount_lock imbalance in path_init()

When backporting commit 4023bfc9f351 ("be careful with nd->inode in
path_init() and follow_dotdot_rcu()"), I failed to account for the
vfsmount_lock that is used in 3.2 but not upstream.  path_init() takes
the lock if performing RCU lookup, but must drop it if (and only if)
it subsequently fails.

Reported-by: nuxi@vault24.org
References: https://bugzilla.kernel.org/show_bug.cgi?id=92531
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
Tested-by: nuxi@vault24.org