introduce fsname_is_auto(name) and FSTYPE__AUTO

The comment in find_fstype_by_name() says:

	just mounting anything is wrong

and this is true in general, but:

	almost every fs has its own features

this is not true in a sense that a lot of supported filesystems do not
need any special processing: FSTYPE__PROC, FSTYPE__SYSFS, and more. More
importantly, this logic does not allow to spicify from the command line
that (say) currently unsupported hugetlbfs can "just work", do_new_mount()
should only pass the right name/options.

This patch adds the new FSTYPE__AUTO code, find_fstype_by_name(name) adds
the new entry if fsname_is_auto(name) returns true. We do not care that
different fstype's can have the same FSTYPE__AUTO code, fstype->code has
no meaning unless we need to do something special with this fs, but in
this case it should not be FSTYPE__AUTO by definition.

Note: currently find_fstype_by_name() just returns true, it is obviously
pointless to "dump" until we teach "restore" to handle FSTYPE__AUTO.
Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>

mount.c

@@ -1172,6 +1172,11 @@ static struct fstype fstypes[32] = {
 	},
 };
 
+static bool fsname_is_auto(const char *name)
+{
+	return false;
+}
+
 struct fstype *find_fstype_by_name(char *_fst)
 {
 	int i;
@@ -1201,13 +1206,22 @@ struct fstype *find_fstype_by_name(char *_fst)
 	for (i = 1; i < ARRAY_SIZE(fstypes); i++) {
 		struct fstype *fstype = fstypes + i;
 
-		if (!fstype->name)
+		if (!fstype->name) {
+			if (!fsname_is_auto(fst))
 				break;
 
+			fstype->name = xstrdup(fst);
+			fstype->code = FSTYPE__AUTO;
+			return fstype;
+		}
+
 		if (!strcmp(fstype->name, fst))
 			return fstype;
 	}
 
+	if (i == ARRAY_SIZE(fstypes)) /* ensure we have a room for auto */
+		pr_err_once("fstypes[] overflow!\n");
+
 	return &fstypes[0];
 }
 

protobuf/mnt.proto

@@ -17,6 +17,7 @@ enum fstype {
 	AUFS			= 13;
 	MQUEUE			= 14;
 	FUSE			= 15;
+	AUTO			= 16;
 };
 
 message mnt_entry {