nfs: Fix use of uninitialized variable in nfs_getattr()

Variable 'err' needn't be initialized when nfs_getattr() uses it to
check whether it should call generic_fillattr() or not. That can result
in spurious error returns. Initialize 'err' properly.

Signed-off-by: Jan Kara <jack@suse.cz>
Cc: stable@vger.kernel.org
Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>