nfs4 use mandatory attribute file type in nfs4_get_root

S_ISDIR(fsinfo.fattr->mode) checks the file type rather than the mode bits,
so we should be checking for the NFS_ATTR_FATTR_TYPE fattr property.

Signed-off-by: Andy Adamson <andros@netapp.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>
Cc: stable@kernel.org