btrfs: Initialize inode::i_mapping once in btrfs_symlink

inode->i_op is initialized multiple times. Perform it once. This was
left by 4779cc04 ("Btrfs: get rid of btrfs_symlink_aops").

Signed-off-by: Goldwyn Rodrigues <rgoldwyn@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>