mount: fill fstypes for btrfs mounts on restore

BTRFS returns subvolume dev-id instead of superblock dev-id,
so we need to know which mounts are btrfs.

The mi->fstype->name is "unsuppoerted" here, because the fstype->code
is saved in an image

{
.name = "unsupported",
.code = FSTYPE__UNSUPPORTED,
},
{
.name = "btrfs",
.code = FSTYPE__UNSUPPORTED,
}

An a second reason is that pocesses can be migrated from smth to btrfs.
This all can happen _only_ for the root mount and for bind mounts of
the root mount...
Signed-off-by: Andrew Vagin <avagin@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>

include/fs-magic.h

@@ -33,4 +33,8 @@
 #define DEVPTS_SUPER_MAGIC	0x1cd1
 #endif
 
+#ifndef BTRFS_SUPER_MAGIC
+#define BTRFS_SUPER_MAGIC	0x9123683E
+#endif
+
 #endif /* __CR_FS_MAGIC_H__ */

mount.c

@@ -25,6 +25,8 @@
 #include "namespaces.h"
 #include "protobuf.h"
 #include "kerndat.h"
+#include "fs-magic.h"
+
 #include "protobuf/mnt.pb-c.h"
 
 /*
@@ -1230,6 +1232,17 @@ static int do_mount_one(struct mount_info *mi)
 	if (ret == 0 && propagate_mount(mi))
 		return -1;
 
+	if (mi->fstype->code == FSTYPE__UNSUPPORTED) {
+		struct statfs st;
+
+		if (statfs(mi->mountpoint, &st)) {
+			pr_perror("Unable to statfs %s", mi->mountpoint);
+			return -1;
+		}
+		if (st.f_type == BTRFS_SUPER_MAGIC)
+			mi->fstype = find_fstype_by_name("btrfs");
+	}
+
 	return ret;
 }