NFS: Remove redundant check in nfs_check_verifier()

The check for nfs_attribute_timeout(dir) in nfs_check_verifier is
redundant: nfs_lookup_revalidate() will already call nfs_revalidate_inode()
on the parent dir when necessary.

The only case where this is not done is the case of a negative dentry. Fix
this case by moving up the revalidation code.

Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>