NFS: Don't clobber the attribute type in nfs_update_inode()

If the NFS_ATTR_FATTR_TYPE field isn't set in fattr->valid, then we should
not set the S_IFMT part of inode->i_mode.

Reported-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>