fs: add the ability for statmount() to report the fs_subtype

/proc/self/mountinfo prints out the sb->s_subtype after the type. This
is particularly useful for disambiguating FUSE mounts (at least when the
userland driver bothers to set it). Add STATMOUNT_FS_SUBTYPE and claim
one of the __spare2 fields to point to the offset into the str[] array.

Reviewed-by: Jan Kara <jack@suse.cz>
Reviewed-by: Ian Kent <raven@themaw.net>
Signed-off-by: Jeff Layton <jlayton@kernel.org>
Link: https://lore.kernel.org/r/20241111-statmount-v4-2-2eaf35d07a80@kernel.org
Acked-by: Miklos Szeredi <mszeredi@redhat.com>
Signed-off-by: Christian Brauner <brauner@kernel.org>

include/uapi/linux/mount.h

@@ -173,7 +173,9 @@ struct statmount {
 	__u32 mnt_root;		/* [str] Root of mount relative to root of fs */
 	__u32 mnt_point;	/* [str] Mountpoint relative to current root */
 	__u64 mnt_ns_id;	/* ID of the mount namespace */
-	__u64 __spare2[49];
+	__u32 fs_subtype;	/* [str] Subtype of fs_type (if any) */
+	__u32 __spare1[1];
+	__u64 __spare2[48];
 	char str[];		/* Variable size part containing strings */
 };
 
@@ -207,6 +209,7 @@ struct mnt_id_req {
 #define STATMOUNT_FS_TYPE		0x00000020U	/* Want/got fs_type */
 #define STATMOUNT_MNT_NS_ID		0x00000040U	/* Want/got mnt_ns_id */
 #define STATMOUNT_MNT_OPTS		0x00000080U	/* Want/got mnt_opts */
+#define STATMOUNT_FS_SUBTYPE		0x00000100U	/* Want/got fs_subtype */
 
 /*
  * Special @mnt_id values that can be passed to listmount