fs: btrfs: Use btrfs_readlink() to implement __btrfs_readlink()

The existing __btrfs_readlink() can be easily re-implemented using the
extent buffer based btrfs_readlink().

This is the first step to re-implement U-Boot's btrfs code.

Signed-off-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: Marek BehĂșn <marek.behun@nic.cz>
diff --git a/fs/btrfs/btrfs.h b/fs/btrfs/btrfs.h
index e819739..53d53f3 100644
--- a/fs/btrfs/btrfs.h
+++ b/fs/btrfs/btrfs.h
@@ -60,6 +60,7 @@
 int __btrfs_lookup_inode(const struct __btrfs_root *, struct btrfs_key *,
 		        struct btrfs_inode_item *, struct __btrfs_root *);
 int __btrfs_readlink(const struct __btrfs_root *, u64, char *);
+int btrfs_readlink(struct btrfs_root *root, u64 ino, char *target);
 u64 __btrfs_lookup_path(struct __btrfs_root *, u64, const char *, u8 *,
 		       struct btrfs_inode_item *, int);
 u64 btrfs_file_read(const struct __btrfs_root *, u64, u64, u64, char *);