nilfs2: make snapshots in checkpoint tree exportable

The previous export operations cannot handle multiple versions of
a filesystem if they belong to the same sb instance.

This adds a new type of file handle and extends export operations so
that they can get the inode specified by a checkpoint number as well
as an inode number and a generation number.

Signed-off-by: Ryusuke Konishi <konishi.ryusuke@lab.ntt.co.jp>

include/linux/exportfs.h

@@ -67,6 +67,19 @@ enum fid_type {
 	 * 32 bit parent block number, 32 bit parent generation number
 	 */
 	FILEID_UDF_WITH_PARENT = 0x52,
+
+	/*
+	 * 64 bit checkpoint number, 64 bit inode number,
+	 * 32 bit generation number.
+	 */
+	FILEID_NILFS_WITHOUT_PARENT = 0x61,
+
+	/*
+	 * 64 bit checkpoint number, 64 bit inode number,
+	 * 32 bit generation number, 32 bit parent generation.
+	 * 64 bit parent inode number.
+	 */
+	FILEID_NILFS_WITH_PARENT = 0x62,
 };
 
 struct fid {