btrfs: remove pointless iocb::ki_pos addition in btrfs_encoded_read()

iocb->ki_pos isn't used after this function, so there's no point in
changing its value.

Signed-off-by: Mark Harmstone <maharmstone@fb.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>