fs_parse: handle optional arguments sanely

Don't bother with "mixed" options that would allow both the
form with and without argument (i.e. both -o foo and -o foo=bar).
Rather than trying to shove both into a single fs_parameter_spec,
allow having with-argument and no-argument specs with the same
name and teach fs_parse to handle that.

There are very few options of that sort, and they are actually
easier to handle that way - callers end up with less postprocessing.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

fs/ceph/super.c

@@ -179,8 +179,8 @@ static const struct fs_parameter_spec ceph_mount_parameters[] = {
 	fsparam_flag_no ("copyfrom",			Opt_copyfrom),
 	fsparam_flag_no ("dcache",			Opt_dcache),
 	fsparam_flag_no ("dirstat",			Opt_dirstat),
-	__fsparam	(fs_param_is_string, "fsc",	Opt_fscache,
-			 fs_param_neg_with_no | fs_param_v_optional, NULL),
+	fsparam_flag_no	("fsc",				Opt_fscache), // fsc|nofsc
+	fsparam_string	("fsc",				Opt_fscache), // fsc=...
 	fsparam_flag_no ("ino32",			Opt_ino32),
 	fsparam_string	("mds_namespace",		Opt_mds_namespace),
 	fsparam_flag_no ("poolperm",			Opt_poolperm),