nfs: fix null checking in nfs_get_option_str()

The following null pointer check is broken.

	*option = match_strdup(args);
	return !option;

The pointer `option' must be non-null, and thus `!option' is always false.
Use `!*option' instead.

The bug was introduced in commit c5cb09b6 ("Cleanup: Factor out some
cut-and-paste code.").

Signed-off-by: Xi Wang <xi.wang@gmail.com>
Cc: stable@vger.kernel.org
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>