f2fs: fix up f2fs_get_parent issue to retrieve correct parent inode number

Test Case:
[NFS Client]
ls -lR .

[NFS Server]
while [ 1 ]
do
echo 3 > /proc/sys/vm/drop_caches
done

Error on NFS Client: "No such file or directory"

When cache is dropped at the server, it results in lookup failure at the
NFS client due to non-connection with the parent. The default path is it
initiates a lookup by calculating the hash value for the name, even though
the hash values stored on the disk for "." and ".." is maintained as zero,
which results in failure from find_in_block due to not matching HASH values.
Fix up, by using the correct hashing values for these entries.
Signed-off-by: Namjae Jeon <namjae.jeon@samsung.com>
Signed-off-by: Amit Sahrawat <a.sahrawat@samsung.com>
Signed-off-by: Jaegeuk Kim <jaegeuk.kim@samsung.com>

fs/f2fs/dir.c

@@ -540,13 +540,13 @@ int f2fs_make_empty(struct inode *inode, struct inode *parent)
 
 	de = &dentry_blk->dentry[0];
 	de->name_len = cpu_to_le16(1);
-	de->hash_code = 0;
+	de->hash_code = f2fs_dentry_hash(".", 1);
 	de->ino = cpu_to_le32(inode->i_ino);
 	memcpy(dentry_blk->filename[0], ".", 1);
 	set_de_type(de, inode);
 
 	de = &dentry_blk->dentry[1];
-	de->hash_code = 0;
+	de->hash_code = f2fs_dentry_hash("..", 2);
 	de->name_len = cpu_to_le16(2);
 	de->ino = cpu_to_le32(parent->i_ino);
 	memcpy(dentry_blk->filename[1], "..", 2);

fs/f2fs/hash.c

@@ -76,6 +76,10 @@ f2fs_hash_t f2fs_dentry_hash(const char *name, int len)
 	const char *p;
 	__u32 in[8], buf[4];
 
+	if ((len <= 2) && (name[0] == '.') &&
+		(name[1] == '.' || name[1] == '\0'))
+		return 0;
+
 	/* Initialize the default seed for the hash checksum functions */
 	buf[0] = 0x67452301;
 	buf[1] = 0xefcdab89;