fat (exportfs): fix dentry reconnection

Maintain an index of directory inodes by starting cluster, so that
fat_get_parent() can return the proper cached inode rather than inventing
one that cannot be traced back to the filesystem root.

Add a new msdos/vfat binary mount option "nfs" so that FAT filesystems
that are _not_ exported via NFS are not saddled with maintenance of an
index they will never use.

Finally, simplify NFS file handle generation and lookups.  An
ext2-congruent implementation is adequate for FAT needs.

Signed-off-by: Steven J. Magnani <steve@digidescorp.com>
Acked-by: OGAWA Hirofumi <hirofumi@mail.parknet.co.jp>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
diff --git a/fs/fat/namei_vfat.c b/fs/fat/namei_vfat.c
index 6a6d8c0..e535dd7 100644
--- a/fs/fat/namei_vfat.c
+++ b/fs/fat/namei_vfat.c
@@ -914,7 +914,7 @@
 	struct inode *old_inode, *new_inode;
 	struct fat_slot_info old_sinfo, sinfo;
 	struct timespec ts;
-	loff_t dotdot_i_pos, new_i_pos;
+	loff_t new_i_pos;
 	int err, is_dir, update_dotdot, corrupt = 0;
 	struct super_block *sb = old_dir->i_sb;
 
@@ -929,8 +929,7 @@
 	is_dir = S_ISDIR(old_inode->i_mode);
 	update_dotdot = (is_dir && old_dir != new_dir);
 	if (update_dotdot) {
-		if (fat_get_dotdot_entry(old_inode, &dotdot_bh, &dotdot_de,
-					 &dotdot_i_pos) < 0) {
+		if (fat_get_dotdot_entry(old_inode, &dotdot_bh, &dotdot_de)) {
 			err = -EIO;
 			goto out;
 		}